proposition: Lemma msetD1K a B : (a +` B) `\ a = B.
proof: Proof. by apply/msetP => x; rewrite !msetE addKn. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap