proposition: Lemma msetDKBC A : cancel (msetD^~ A) (msetB^~ A).
proof: Proof. by move=> B; rewrite msetDC msetDKB. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap