proposition: Lemma msetBDK A B : A `<=` B -> B `\` A `+` A = B.
proof: Proof. by move=> /msubsetP AB; apply/msetP => a; rewrite !msetE subnK. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap