// For reproducing emacs issue of slow response in C# mode.
// Ignore any swearing. 

using Antlr4.Runtime.Tree;  // faq - need this for ParseTreeProperty
using System;
using System.Collections.Generic;
using System.Text;

using static LDB.LDButils;
using static LDB.TypesBroadly;
using static LDB.InstFullyOrPartly;
using static LDB.errorHandling;
using static LDB.SymTab;

using System.Linq;
using System.Diagnostics.CodeAnalysis;  // where from? why? - todo

using static LDB.globals;


// Caused trouble but was generally worth it; caught a few bugs and
// will probably catch a few more. Yep, definitely worth it!
//
//
// Certain methods/properties (e.g. the name property) were declared
// in abstract superclasses but when I added '#nullable enable' they
// started reporting nullability errors. I could either disable those
// with "= null!", totally defeating the point, or deal with them
// properly by pulling those methods/properties down into the concrete
// subclasses, which was more verbose and in some sense less
// encapsulated. In the main, I chose to do that to get static
// guarantees.
#nullable enable


/*

This module is called expressions because that's where I started, and
SQL at heart is an expression language, however there will be
non-expression constructs such as if and loops, and they go here as
well.

*/

namespace LDB {

	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj




	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj



	public interface IhandlePrefixes<T>
						where T : LDBRoot {

		public bool isPrefixed { get; }

		public bool isExcessPrefixed { get; }

		// The T in addPrefix/rePrefix/withoutPrefix is because we may be 
		// returning eg. a list of items with prefixes 
		// thus modified, not a single PMQIdent

		public T addPrefix(PMQIdent pfx);

		public T rePrefix(PMQIdent newpfx);

//		public T stripPrefix(PMQIdent id);
	}
	// todo - move this





	public abstract class LDBRoot {
		// Root of everything LDB-ish

		public bool Equals([AllowNull] LDBRoot other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals on LDBRoot");
		}

		public override
			bool Equals([AllowNull] Object other) {
			throw new LDBShouldNotBeImplementedException(
									"called Equals in Object");
		}

		public override int GetHashCode() {
			throw new LDBShouldNotBeImplementedException(
									"called GetHashCode in LDBRoot");
		}

		public static bool operator ==(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called == in LDBRoot");
		}

		public static bool operator !=(LDBRoot lhs,
									   LDBRoot rhs) {
			throw new LDBShouldNotBeImplementedException(
									"called != in LDBRoot");
		}


		public virtual string className {
			// todo - this needs checking it works as I want
			// also rename to ldbclassname -- todo
			get => this.GetType().FullName ?? "(unknown)";
		}


		public virtual string classNameQ =>
			"'" + className + "'";


		public virtual string classNamePlus(string plus) =>
			className + "." + plus;

		public override string ToString() {
			// This implicit to-string conversion crap cost me a few hours, so at least
			// do this & blow up at runtime. Would prefer compile time but it seems
			// not possible.  This turns out to have been a good idea,
			// saved me quite a bit more debugging!
			// <https://stackoverflow.com/questions/15345517/is-it-possible-to-disable-implicit-tostring-call>
			// Will try <https://stackoverflow.com/questions/54354342/how-to-get-an-error-shown-when-using-non-strings-as-strings-instead-of-an-autom/54355115#54355115)
			var msg = "LDBRoot.ToString()";
			throw new LDBToStringImplictConversionException(msg);
		}

		public abstract string toDebugStr();
	}



	public abstract class Ephemeral : LDBRoot {
		// for objects that are not part of the AST
	}



	public interface IWithLineNUm {  // todo - use or remove - todo
		//	public 
	}


	public interface IHasTableAlias {
		// Has a table alias AST obj. May be a ...Missing subtype in which
		// it has no alias given by the user, but alias AST is there. More
		// accurately a TableSource alias but ok
		public abstract TableAlias ta { get; set; }
	}


	// Base class for all LDB AST objects
	public abstract class LDBASTobj : LDBRoot {
		// This one is a problem. If I declare the 'parent' property
		// abstract I then have to implement it literally hundreds of
		// times in the derived classes, and I really don't think it
		// would help so just set it to a default which gets overwritten later.
		// 
		// The problem is that those objects were created by the
		// parser, which creates them bottom-up, which means you can't
		// know the parent when they are created. It should be
		// possible to work around this but it does mean reworking the
		// parser fundamentally, and I'm not going to do that
		// assumeing it's even possible.

		// todo - read up on priv/pub/prog accessors I really don't
		// get these at all
		public LDBASTobj parent { get; private set; }
						= noParent;
		// Unique ID per object, useful for distinguishing them
		// without general equality.
		readonly public ulong uid = newUID();

		// public abstract int lineNum { get; set; }

		// Is validated is a cheapish check to ensure everything gets
		// validated
		// can do this as auto prop? todo
		private bool _validated = false;

		protected bool validated {
			get => _validated;
			set {
				// hardMust(!validated, "... already validated... "); - no
				// It's hard to avoid validating a thing twice, so for now allow it
				// Look into it later, really shouldn't happen
				_validated = true;
			}
		}

		public LDBStructsChildren kids { get; private set; } =
					new LDBStructsChildren(noParent); // just easier

		public LDBASTobj() {  //////////////////////////////////////////////////////////////// todo dead?
							  // VStudio thinks this has zero references, which is true explicitly, but it's 
							  // called implicitly on every obj creation - or should do todo- check
			if (recordObjectsCreated) {
				AllCreatedItems.Add(this);
			}
		}


		public virtual void validate() {
			if (reportAlsoIfValidationOK) {
				debugmsg("validating..." + className
					+ " (" + asNameOrIdentContents() + ")");
			}
			mhp_mhk();
			kids.validate();
			// validated = true; -- No, get each subclass to set
			// validated. That forces an explicit call to base() which
			// is more code but ok.
		}


		public bool isValidated { get => validated; }  // todo just => validated;


		public virtual bool isTopLevelItemSeq => false;


		public virtual void mhp_mhk() {  // don't think this need be virtual - todo?
										 // = must have parent, must have kids. The kids may be the
										 // empty set and the parent may be noParent but they must
										 // exist.
										 // Why not just inline this into validate()?
										 // Because a few classes need bespoke validation, but still must check this.
			hardMustParentIsPresent();
			kids.eachKidParentMustBe(this);
		}


		public string asNameOrIdentContents() {
			// for very crude debugging 
			var optName = this is IisNamedItem ni ? ni.name.toRawStr() : "";
			optName = (optName == "" && this is PMQIdent) ? this.toRawStr() : "";

			var optName2 = optName + "  (uid:" + uid.ToString() + ") "
				+ getFirstFewCharsOf(toRawStr());
			return optName2;
		}


		public virtual void getAllUIDs(UIDset uids) {
			uids.Add(this);
			kids.ForEach(namedKid => namedKid.kid.getAllUIDs(uids));
		}


		public bool isStubParent { get => (this is NoParent) || (this is FakeParent); }


		public void validateSpecificItems(params LDBASTobj[] items) {
			Array.ForEach(items, item => item.validate());
		}


		public T? optClimbTo<T>() where T : LDBASTobj {
			var resItem = this;
			while (!(resItem is T)) {
				if (resItem.parent.isStubParent) {
					return null;
				}
				resItem = resItem.parent;
			}

			var res = resItem as T;
			return res;
		}


		public T reqClimbTo<T>() where T : LDBASTobj {
			var loc = className + ".reqClimbTo<T>()";
			var res = optClimbTo<T>();
			hardMust2(!(res is null), 
				// Thanks SO <https://stackoverflow.com/questions/2581642/how-do-i-get-the-type-name-of-a-generic-type-argument>
				// Todo - use this elsewhere, poss ExpectException? todo
				() =>
				loc 
				+ nl + ", climbed to top, didn't find type "
				+ typeof(T).FullName
				+ nl + "Hierarchy is: "
				+ nl + getItemWithParents(this));
			if (res is null) { throw new NNQC(); }
			return res;
		}


		public void hardMustParentIsPresent() {

			hardMust2(!(parent is null), 
				() =>
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");

			hardMust2(!(this.parent is NoParent),
				() =>
				"Parent is NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public void hardMustParentIsAbsent() {
			// For checking cloning - parent should be NoParent immediately 
			// after, before it's set by parent, and never null
			hardMust(!(parent is null),
				 "NULL parent for item of type\n"
				 + $"{GetType()}\n.");
			hardMust((this.parent is NoParent),
				"Parent should be NoParent on object of type\n"
				+ $"'{this.className}'");
		}


		public bool hasParent<P>() {
			hardMust(!(this is NoParent), "Item is NoParent");
			return this.parent is P;
		}


		public bool hasAncestor<A>() {
			hardMust(!this.isStubParent, "Item is NoParent");

			// how to do with a switch expr?
			if (this.parent.isStubParent /* is NoParent || this.parent is FakeParent*/ ) { // todo do as x.hasFudgedParent() - todo
				return false;
			} else if (this.parent is A) {
				return true;
			} else {
				return this.parent.hasAncestor<A>();
			}
		}

		public bool notHasAncestor<A>() =>
			!hasAncestor<A>();


		public void setMyParentTo(LDBASTobj prnt) {
			// An object may only set its parent if its current parent is
			// the dummy 'noParent', which almost every object's parent is
			// set to on creation (see LDBASTobj parent property def).

			// The test for null is peculiar given this. Because
			// noParent is created statically, setting the parent of
			// noParent is difficult, perhaps impossible, to do statically
			// and reliably.Despite the declaration '#nullable enable', it
			// gets created with a null parrent (quite reasonably), hence
			// this check.The actual setting of the parent has to be done
			// at runtime i.e.after the static creation of 'noParent'.

			hardMust2((this.parent is NoParent) || (this.parent is null),
				() =>
				"tried to re-set parent on object of type: "
				+ nl + this.classNameQ
				+ nl + "Original parent type is: "
				+ (this?.parent?.classNameQ == null ? "null" : this.parent.classNameQ)
				+ nl + "and new parent type is: "
				+ prnt.classNameQ
				+ nl + "Item having parent set on it is: "
				// because object may not be fully constructed yet, check howver TODO this is a major hack and must be fixed sometime.
//				+ nl + (this is null ? "(null)" : this.toRawStr()));
				+ nl + this.toRawStr() );

			parent = prnt;
		}


		public void forceNewParentForRootObject() {
			// Every object must have some parent, including the
			// root object of the parse, which is LDBitems
			// (a sequence of LDB items), so this forces one.
			// It is only for that purpose!
			// 
			// LDBitems obj exists for sub objs such as the statements
			// in a while loop, in a proc, in an if... etc., for
			// which their parent is the while/proc/if etc.
			hardMustParentIsAbsent();
			this.parent = fakeParent;
		}


		//public void resetMyParentTo(LDBASTobj prnt) {  ////////////////////////////////// destroy!!!!!! todo
		//	parent = prnt;
		//}

		//////////////////////// xxxxxxxxxxxxx delete - todo
		//private void setParentOnKid(LDBASTobj kid) => // todo needed?
		//											  // check parente doesn't exist - todo
		//	kid.parent = this;

		public virtual void setParentOnKidsAndMakeKidsDict(  // todo - and add to kids dict
															 // this mixes up setting parent and making child dict. Not good. - todo
						params (string kidName, LDBASTobj kid)[] kidsIn) {
			var loc = classNamePlus("setParentOnKidsAndMakeKidsDict");

			var kidsToAdd = new LDBStructsChildren(this, kidsIn);
			hardMust2(kids.isEmpty,
				() =>
				$"setting kids on parent '{className}'" 
				+ "when kids is not empty, "
				+ $"kids is ({kids.Count} kids):"
				+ nl + kids.toDebugStr()
				+ nl + $"Trying to add\n{kidsToAdd.toDebugStr()}\n"
				+ $"in {className}");
			// if there is an LDBStructsChildren and another child,
			// this will blow up - will need to do kids.Add or
			// something - in fact, kids needs to be created in one
			// go, which is what happens.
			kids = kidsToAdd;    ///////////////////////////////////////////////////////// why this? todo
			Array.ForEach(kidsIn, xkid => hardMust(!(xkid.kid is null), loc, "null kid"));
			Array.ForEach(kidsIn, xkid => xkid.kid.setMyParentTo(this));
		}


		public virtual void setParentOnKidsAndAddToKidsDict(
					params (string kidName, LDBASTobj kid)[] kidsIn) {
			var kidsToAdd_justForErrReporting = new LDBStructsChildren(this, kidsIn);
			hardMust2(!kids.isEmpty, // may disable this later 
				() =>
				"adding to kids when kids is empty, "
				 + $"kids is\n{kids.toDebugStr()}\n"
				 + $"Trying to add\n{kidsToAdd_justForErrReporting.toDebugStr()}\n"
				 + $"in {className}");
			Array.ForEach(kidsIn, xkid => {
//				setParentOnKid(xkid.kid);
				xkid.kid.setMyParentTo(this);
				kids.Add(xkid);
			});
		}


		public virtual IdentsUsed getAllIDs() {
		/* This is overridden only in TemplatedAgg, and that's only to 
			deliberately fail. There's something wrong there, todo - 
			un-virtualise this and remove the one override. - todo
		*/
			var iu = new IdentsUsed();
			this._getAllIDs(iu);
			return iu;
		}

		public virtual void _getAllIDs(IdentsUsed iu) {
		// Gets all the identifiers explicitly used (read or written)  ----- todo update this
		// anywhere in the AST.  Does not get idents which are
		// declarations ie. parameters, or var/const declarations
		// unless these are combined with an assignment.
		//
		// Implied idents such as those from tbl.* are implicit and ignored.
		//
		// Root caller supplies new, empty iu
		// need usage here - todo docs//
			kids._getAllIDs(iu);
		}

		public abstract bool canGetAllIdentsUsed {
		// 	This seems to be true on most classes so I could set true 
		// as a default here and override it where it's not wanted, saving 
		// a chunk of code, but I'd rather make it explicit to force me to consider each case. A bit more C&P for less chance of bugs.
			get;
		}


		// Produces a string that should be identical to (excepting
		// whitespaces) the input ie. emit LDB code not valid SQL
		//
		// ToString was causing too many weird problems (due to my
		// inexperience with c#) so just used toRawStr()
		public abstract string toRawStr();

		public override string toDebugStr() => toRawStr();

	} // end LDBASTobj


	
}