  Just as binary relations between sets may be understood as jointly monic
spans, so too may equivalence relations on the disjoint union of sets be
understood as jointly epic cospans. With the ensuing notion of composition
inherited from the pushout of cospans, we call these equivalence relations
\emph{corelations}. We define the category of corelations between finite sets
and prove that it is equivalent to the prop for extraspecial commutative
Frobenius monoids. Dually, we show that the category of relations is equivalent
to the prop for special commutative bimonoids. Throughout, we emphasise how
corelations model interconnection.
