[Xy-pic] xy-pic arrow problem

[Philip Hirschhorn]

I can't explain the weird error message, but the problem seems to be
that you've left out the ampersand that should separate the nodes.

That is: your

  $\xymatrix{R\ar@{-->}[r] B}$

should be

  $\xymatrix{R\ar@{-->}[r] &B}$

In case it matters, I loaded xy-pic with

  \usepackage[all,cmtip]{xy}

Phil Hirschhorn