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