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