New coordinate formats

??=[coord] it is possible within entries to refer to all the entries of the -matrix using the following special <coord>inate forms:
\begin{defs} \vert''\vert$r$\vert,\vert$c$\vert''\vert & Position and extents o... ...f \vert dulr\vert describing one \lq move' to a neighbor entry \par \cr \end{defs}
So the current entry has the synonyms |[0,0]|, |[]|, |[rl]|, |[ud]|, |[dudu]|, etc.

These forms are useful for defining diagrams where the entries are related, ,
\begin{code} \begin{displaymath}\xy \xymatrix{ A \POS[];[d]**\dir{~}, [];[dr]**\dir{-} \\ B & C \POS[];[l]**\dir{.} } \endxy\end{displaymath}\end{code}
was typeset by

If an entry outside the -matrix is referenced then an error is reported.

2 These handle the parsing and interpretation of bracketed <coord>inates; the number is computed already at parse time:

( @@@ =

@@[@[##1]@[##1]@@i##1]@ =@ @

@@i@=@ @@=@ A@ =@letter ]@ =@letter @ =@other @

@@letteru@@ne @ u@@letterd@ne @ d@@letterl@@@ne @ l@@letterr@@ne @ r@@letter]@]@x@ =@letter @illegal <coord>: only letters lrud within []s@

@@other#1,#2]@#1@@#2@x )

The next does the actual lookup at -time where we can still print an error message using the saved original form.

( @@x@##1@@@ Q@##1 @ @RC##1@ @@ @

@@RC#1@in entry "@,@": No @@ (is "#1") from here )

3