[Xy-pic] bug?

[James Borger]

Dear xy-pic-ers,

I've found what appears to be a bug in the way xy-pic handles 
intersections of lines.  That said, I have pretty limited experience with 
xy-pic, so the bug could easily be in me.

I want to typeset a three-dimensional commutative diagram and have some
lines appear to pass under others.  The problem is that sometimes xy-pic
draws only one of the two pieces of the lower line, which should be
broken.  I had this problem once before and was able to work around it by
changing things that I though should have no effect on the output.
Unfortunately, I didn't save that example.

The misbehaving (LaTeX) code is below (in simplified form).  The arrow
from J to K is the offending one.

Best,

Jim Borger

=======================================================================

\documentclass{amsart}
\usepackage{amscd, amssymb}
\usepackage[all]{xy}

\begin{document}

\[
\begin{xy} <0pt,0pt>;<16mm,0mm>:<0mm,-7mm>::
  (2,0) *+{A}="a",
  (5,0) *+{B}="b",
  (2,3) *+{C}="c",
  (5,3) *+{D}="d",
  (0,4) *+{E}="e",
  (2,6) *+{F}="f",
  (5,6) *+{G}="g",
  (0,7) *+{H}="h",
  (3,7) *+{I}="i",
  (1,8) *+{J}="j",
  (4,8) *+{K}="k",
  (0,10) *+{L}="l",
  (3,10) *+{M}="m",
  \ar "a";"b"
  \ar "c";"a"
  \ar "d";"b"
  \ar "e";"a"
  \ar "f";"c"
  \ar "f";"g" |!{"i";"d"}\hole
  \ar "g";"d"
  \ar "h";"e"
  \ar "h";"i"

  \ar "i";"d"
  \ar "f";"j" |!{"h";"i"}\hole
  \ar "j";"k" |!{"m";"i"}\hole
  \ar "k";"g"
  \ar "l";"j"
  \ar "l";"h"
  \ar "l";"m"
  \ar "m";"i"
  \ar "m";"k"
\end{xy}
\]

\end{document}