proposition: Lemma msetUC (A B : {mset K}) : A `|` B = B `|` A.
proof: Proof. by apply/msetP => a; rewrite !msetE maxnC. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap