proposition: Lemma msetDA A B C : A `+` (B `+` C) = A `+` B `+` C.
proof: Proof. by apply/msetP=> x; rewrite !msetE addnA. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap