[Xy-pic] an arrow with a tip in the middle

[Rafael Villarroel]

Dear list,

When drawing an \xygraph, I would like to have an arrow with a "tip"
in the middle.
If the "direction" is horizontal, I can somewhat fake it, as in the
"arrow" from A to B.
But in other directions it just does not work, as in B to C.  Any suggestions?
TIA

Rafael

\documentclass{article}
\usepackage[graph]{xy}

\begin{document}
\xygraph{
   !~F{[]*+<2.5pt>[o]@{*}}
   [] !F="A"
   [rr] !F="B"
   [ur] !F="C"
   "A"-|{>}"B"
   "B"-|{>}"C"
}

\end{document}