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