proposition: Lemma in_msetD A B a : (a \in A `+` B) = (a \in A) || (a \in B).
proof: Proof. by rewrite !in_mset !msetE addn_gt0. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap