[Xy-pic] Trouble with monomorphisms

[Sesquipedalian Abecedarian]

The xyguide (p. 8) says to use a space to typeset a monomorphism nicely:

\xymatrix{ A \ar @{>->} @< 2pt> [r] \ar @{ >->} @<-2pt> [r] & B }

But on my system (Debian stable tetex), this generates an error:

! Xy-pic error: illegal <dir>: \dir { >} not defined.
\xyerror@ ...#2}\fi \errmessage {Xy-pic error: #1}
                                                  }
l.1597 ...< 2pt> [r] \ar @{ >->} @<-2pt> [r] & B }

I actually want to typeset a vertical monomorphism, so simply adding
space to the vertex label doesn't work.

Any ideas?  Thanks in advance.

Xy-pic version 3.7 <1999/02/16>
Copyright (c) 1991-1998 by Kristoffer H. Rose <krisrose at ens-lyon.fr>
Xy-pic is free software: see the User's Guide for details.