proposition: Lemma fsetISS A B C D : A `<=` C -> B `<=` D -> A `&` B `<=` C `&` D.
proof: Proof. by move=> /(fsetSI B) /fsubset_trans sAC /(fsetIS C) /sAC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)