proposition: Lemma msetSU C A B : A `<=` B -> A `|` C `<=` B `|` C.
proof: Proof. by move=> sAB; rewrite -!(msetUC C) msetUS. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap