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