[Xy-pic] Math fonts

[R.P.I. Lewis]

Yannick Delbecque <yannick.delbecque at mail.mcgill.ca> writes:

> After a few tests, I get the same error with the minimal file:
>
> \documentclass{article}
> \usepackage[all]{xy}
> \CompileMatrices
> \begin{document}
> \[ \xymatrix{ \mathrm{R} } \]
> \end{document}
>
> But there is no error if I desactivate "\CompileMatrices" or remove
> the \mathrm command!
>

I too see this behaviour. If you enclose \mathrm{R} in a {...} pair
then it works fine.  Basically the problem is that the first cahracter
of a vertex has to be non-expandable, or the compilation process
breaks---i've not yet found a matric that cant be compiled by
enclosing all its veertices in {...}.  If the thing still wont
compile, or you object to adding the braces, then change \xymatrix to
\xymatrixnocompile, and \Xy-pic will not attempt to compile that
particular matrix (this also works in your example above).

  I learned all this from section 1.7 of the Xy-pic users guide (page
4).