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