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