[Xy-pic] Problem with skewing(?)

[Richard Lewis]

Hi everyone,

I am trying to define a command \SliceObj (that will take three
arguments: top, label and bottom) to typeset a vertical arrow, framed with parentheses (...),
whose reference point should be the middle of the label.

Something like
           
   /       \
  |  {#1}   |
  |   |     |
  |   |{#2} |
  |   |     |
  |   V     |
  |  {#3}   |
   \       /

with the reference in the middle of {#2}

I can draw the arrow and frame it (See attached .tex file), and I
thought i could use the skewing operator (!) to move the reference
point to the new positon:

"arrow-with-frame"!<new position>

However I can't get this to work: the position we end up after the
skew is not <new position>, as (i think) the attached file shows, what
am i doing wrong?

(This happens with or without the xymods.tgz patches, so presumably I
have misunderstood something about the skewing operation?)


Worse, trying to use my attempt inside an \xygraph can produce
infinite loops (two examples of this can be found, commented out, in
the attached file)

Any help gratefully appreciated!

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