proposition: Lemma msetDKB A : cancel (msetD A) (msetB^~ A).
proof: Proof. by move=> B; apply/msetP => a; rewrite !msetE addKn. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap