proposition: Lemma setSI A B C : A \subset B -> A :&: C \subset B :&: C.
proof: Proof. by move=> sAB; rewrite -!(setIC C) setIS. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)