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