Re: [Lilypond-auto] Issue 4083 in lilypond: \keepWithTag does not nest well

[lilypond]

Comment #2 on issue 4083 by address@hidden: \keepWithTag does not nest well
https://code.google.com/p/lilypond/issues/detail?id=4083

> > Should be pretty robust and fully backwards-compatible but
> > relies on global side effects.

> What would these be?  Would they be serious?

Any "tag layer" needs to be defined globally.  Relying on global  
definitions/variables is bad for arranging stuff in new ways.

> > b) \keepWithTag only removes tagged music without the given
> > tag(s) in sequential or parallel music only when one of the
> > named tags is actually present on one of the other elements
> > of the list.

> What list?

List of music expressions.  Like with << music music... >> or { music  
music... } where we have a list of music expressions.  So if I have
<< \tag xxx \musicA
   \tag yyy \musicB
   \tag zzz { \tag kkk { \tag ccc c' \tag xxx d' } \tag lll <c d e> } }

then \keepWithTag zzz will remove \musicA and \musicB but keep the rest.   
\keepWithTag ccc will remove the \tag xxx d' but nothing else.  The problem  
with this approach is that stuff like \relative c' or \transpose c d or  
similar, applied immediately  to a tagged expression, will move that  
expression out of sight, so if it is tagged with \tag xxx and \keepWithTag  
xxx is called on the surrounding expressions, any \tag yyy or \tag zzz that  
were at the same layer before \transpose moved the tag out of their  
vicinity will be retained.  So
<< \tag xxx ...
   \tag yyy ...
   \transpose c d \tag zzz ...

would not change at all after \keepWithTag zzz.  Not sure how back the  
drawback is and whether one should try being smarter.  But then we are  
getting into c) territory rather fast.

I don't understand this, maybe because of the two "only"s.
I'm still trying to work out whether they have a subtle
meaning when present together or whether it's just a typo.


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