This is a set of macros that are a front end to xy-pic that make it
(relatively) easy to use it to set the kind of diagrams that are
especially interesting to category theorists.  There are two modes of
macros: the first kind uses shapes such as squares and various triangles
that fit together like Lego blocks; the second sets the nodes on an
xy-grid and labels them and then sets arrows between the nodes.  All the
arrow types of xy are allowed in either mode.   For the latest upgrade
(if any) look at http://www.math.mcgill.ca/barr/papers/.