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