[Xy-pic] pullback symbols

[Andrej Bauer]

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:

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

Andrej