proposition: Lemma subUset A B C : (B :|: C \subset A) = (B \subset A) && (C \subset A).
proof: Proof. by rewrite -setCS setCU subsetI !setCS. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)