proposition: Lemma msetUSS A B C D : A `<=` C -> B `<=` D -> A `|` B `<=` C `|` D.
proof: Proof. by move=> /(msetSU B) /msubset_trans sAC /(msetUS C)/sAC. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap