RE: [Groff] .ch request query

[Ted Harding]

On 20-Aug-03 Dean Allen Provins wrote:
> Hello:
> 
> I had reason to place two traps at the same location, and as ".wh" at
> the same place causes the second to overwrite the first (or so I
> understood), I created the second at another trap location, and tried
> to "move" it to the first.  This seemed not to work with groff version
> 1.19.  Example code follows, and the postscript output is attached.

Hi Dean!

Nothing wrong with what you've done except that you've "understood" it
the wrong way round (and very understandably: the way it really works
is the opposite of what you'd intuitively expect, and it's confusing
on first reading).

Quote from "Troff Users Manual" (section 7.5):

  "Two traps may be planted at the same position only by first planting
   them at different positions and then moving one of the traps. The
   _first planted_ trap will conceal the _second_ unless and until the
   first one is moved (see Tutorial Examples). If the first one is moved
   back, it again conceals the second trap."

The relevant Example seems to occur in the example section on footnotes,
where there is code

  .wh 12i fo     \"footer trap -> temp pos
  .wh -\\nbu fx  \"fx at footer position
  .ch fo -\\nbu  \"conceal fx with fo

In this case the example fails to resolve an apparent ambiguity, since
trap 'fo' is _both_ the first trap planted, and _also_ is moved to the
position of the trap 'fx', so this could intuitively be interpreted as
moving 'fo' "on top of" 'fx'. But according to the citation, it is the
first planted trap which stays "on top" and, if the second planted one
is moved to its position, then it is moved "underneath" the first!

The behaviour you observe (which also occurs with earlier versions of
groff) is consistent with the interpretation I just gave (which I once
had to suss out by trial and error!). In fact, "moving" is not "planting".

In other words: you plant trap 1 somewhere, and then trap 2 somewhere
else. If you move 1 to the position of 2 then it goes "on top of" 2.
If you move 2 to the position of 1 then it goes "underneath" 1.
Motto: in case of conflict netween traps in the same position, the
earliest trap planted anywhere is always on top, even if you have moved
it.

In your code, reverse the order of planting 'x' and 'y', but still
move 'y' to the position of 'x': you will see that you now get the
macro "y" invoked, where you got "x" before.

(And you can test the general rule by defining a 3rd macro "z" like
"x" and "y", and planting trap 'z' with

  .wh 4i z

Then no matter how you shuffle the order of

  .wh 2i x
  .wh 3i y
  .wh 4i z

and of

  .ch y 2i
  .ch z 2i

it is always the earliest ".wh ..." which is "on top".)

Best wishes,
Ted.


--------------------------------------------------------------------
E-Mail: (Ted Harding) <address@hidden>
Fax-to-email: +44 (0)870 167 1972
Date: 20-Aug-03                                       Time: 22:37:33
------------------------------ XFMail ------------------------------