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