# [Xy-pic] bug?

James Borger borger@math.uchicago.edu
Fri, 30 Nov 2001 12:17:09 -0600 (CST)

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}