[Xy-pic] Xypic infinite loop

[Richard Garner]

Dear all,

Below is a minimal example of a tex file which seems to send xypic into an 
infinite loop, leading to a stack overflow.

----

\documentclass{article}
\usepackage[arrow, matrix]{xy}

\begin{document}
\[
     \xymatrix{
     **{!/l 10em/} a \ar[r] & b
     }
\]
\end{document}

----

The problem seems to go away if:

- you remove the \ar[r];
- you change the /l 10em/ to any other <vector> (cf. xyrefer, p.8);
- you add the option \CompileMatrices to the preamble.

I think this may be a bug in xypic. Does anyone have any 
suggestions?

Thanks,

Richard