proposition: Lemma allpairs_tupleP f t (u : m.-tuple U) : @size rT (allpairs f t u) == n * m.
proof: Proof. by rewrite size_allpairs !size_tuple. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq choice fintype path)