proposition: Lemma msetUUl A B C : A `|` B `|` C = (A `|` C) `|` (B `|` C).
proof: Proof. by rewrite msetUA !(msetUAC _ C) -(msetUA _ C) msetUid. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap