[Tex/LaTex] Showing arrows crossing over or under other arrows in xy-pic without knots

Question

I have an xymatrix in which two arrows cross each other, say

\xymatrix{ \ar[dr] &  \\ \ar[ur] & }

and I would like one arrow to cross over the other. I also want to to be able to do this without using knots; the reason is that I want to draw some diagrams where the arrows just intersect and some diagrams where they don't, and I'd prefer that they have a uniform look to them.

There is an example of how to do this in the xy-pic user's guide, but 1) I don't quite understand it, and 2) it seems to require that I thread a single arrow through multiple entries. I don't want to have to do that; I really just want two arrows, one of which crosses over the other.

I tried to give the "under" arrow an empty label, but that didn't work. Any ideas?

Answer 1

It's not so hard.

Dig a hole:

\xymatrix{ \ar[dr]|\hole &  \\ \ar[ur] & }

Calculate the intersection point and dig a hole:

\xymatrix{ \ar[drr]|!{[d];[r]}\hole & & \\ \ar[ur] & & }