proposition: Lemma msetDCA A B C : A `+` (B `+` C) = B `+` (A `+` C).
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