[Xy-pic] arrow shape for relations

Jonas Frey jonas743 at gmail.com
Tue Jan 13 16:19:43 CET 2009

Dear Developers,

I would like to use arrows of shape
in xymatrix to represent relations and profunctors.

If I read the documentation right, that's not possible with
\ar@{ },
because arrows are always of the form tail/stem/head and thus we can't put 
anything "in the middle".
However, in section 23 of the reference manual, I saw an example of the \path 
feature that looked similar.
I tried to use that, but finally gave up because explanation was too technical 
for me to comprehend, and I decided to ask for help here.

My precise question is:
How to do a macro for arrows of the shapes given above that I can place in 
\xymatrix, and that behaves similar to
\ar ?
Ideally, I would like to use coordinates and put labels in a way similar to 

Any hints would be really appreciated!



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