proposition: Lemma fsetSI A B C : A `<=` B -> A `&` C `<=` B `&` C.
proof: Proof. by move=> sAB; rewrite -!(fsetIC C) fsetIS. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)