# [Xy-pic] pullback symbols

Andrej Bauer Andrej.Bauer@andrej.com
30 Jul 2001 15:49:12 +0200

Don MacInnes <macinndx@cs.man.ac.uk> writes:
> I'm hoping someone can help by telling me how to draw the little
> right-angle' device often used to show that a certain commuting square is
> a pullback/pushout using xypic.

I have this in my macros.tex and I do not know where I got it from
(neither do I understand what precisely it does):

\newcommand{\pullbackcorner}[1][dr]{\save*!/#1+1.2pc/#1:(1,-1)@^{|-}\restore}
\newcommand{\pushoutcorner}[1][dr]{\save*!/#1-1.2pc/#1:(-1,1)@^{|-}\restore}

Example of use:

$$\xymatrix{ A \ar[r] \ar[d] \pullbackcorner & B \ar[d] \\ C \ar[r] & D \pushoutcorner }$$

Andrej

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