proposition: Lemma msetDAC A B C : A `+` B `+` C = A `+` C `+` B.
proof: Proof. by rewrite -!msetDA (msetDC B). Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap