# [plt-scheme] likely 3.99.0.22 -> 3.99.0.23 incompatibility: hashtables

Jos Koot wrote:
>* I always thought a relation was defined as a subset of the product of
*>* two (or even more) sets (not necessarily different ones, of course)
*>* Jos
*
They're equivalent. A map is a function is a subset of A x B with
certain restrictions (for all a in A, there is exactly one b in B where
(a,b) is in the function; partial function is "at most one b in B"). We
can regard a relation, a subset of A x B, as a function from A onto the
power set of B such that the function returns every b in B so (a,b) is
in the relation.
Graham