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