proposition: Lemma subUsetP A B C : reflect (A \subset C /\ B \subset C) (A :|: B \subset C).
proof: Proof. by rewrite subUset; apply: andP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)