[Xy-pic] Doubts about syntax: can <cs> and <pos> expand to <empty>?

[Eduardo Ochs]

Hi List,

I am trying to draw syntax/parse trees for some XyPic code that I
found in the diagxy manual, but there are some things that don't make
sense to me yet. I tried to parse this code myself,

  \POS(1500,1000)*+!!<0ex,.75ex>{T_{n-1}}
  \ar@{-}|!{(1000,500);(1500,500)}\hole(1167,334)
  \POS(1167,334)
  \ar|!{(1000,500);(1000,0)}\hole_<>(.6){h_{n-1}}
  (500,0)*+!!<0ex,.75ex>{S_n}

but I only managed to write parse trees for its second line if both
<cs> and <pos> can expand to <empty>...

My trees are here,

   http://angg.twu.net/LATEX/2019xysyntax.pdf (PDF)
   http://angg.twu.net/LATEX/2019xysyntax.zip (source)

and if anyone want to look at the source code for that, try:

  rm -rfv /tmp/2019xysyntax.zip
  rm -rfv /tmp/edrx-latex/
  cd /tmp/
  wget http://angg.twu.net/LATEX/2019xysyntax.zip
  mkdir    /tmp/edrx-latex/
  unzip -d /tmp/edrx-latex/ /tmp/2019xysyntax.zip
  cd       /tmp/edrx-latex/
  lualatex 2019xysyntax.tex

I think you'll be amused by the way that the syntax trees are drawn in
the source, using ascii art in comment blocks. These comments are
parsed by Dednat6, that is included in the .zip; Dednat6 is a
front-end for diagxy and proof.sty, and diagxy in its turn is a
front-end for XyPic. The main links for Dednat6 are:

  http://angg.twu.net/LATEX/2018tugboat-rev1.pdf (TUGBoat article)
  http://angg.twu.net/LATEX/2018dednat6-extras.pdf (extra features/docs)
  http://angg.twu.net/dednat6.zip (zip file)
  http://angg.twu.net/dednat6.html (home page)
  http://tug.org/pipermail/luatex/2019-January/007034.html (a question)

Cheers & thanks in advance =),
  Eduardo Ochs
  http://angg.twu.net/
  http://angg.twu.net/math-b.html
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