relations f and g (which are monomorphisms
f : X >-> A x B and g : Y >-> B x C in *Sets)
to the composite relation h : Z >-> A x C ?
It would be interesting if one could form
relations on arbitrary categories and compose
them. I suppose a corelation would be an
epic from a coproduct to an arbitrary object,
like A + B ->> X . It works sort of like
identification of objects of A and B (??)