proposition: Lemma fsetUUl A B C : A `|` B `|` C = (A `|` C) `|` (B `|` C).
proof: Proof. by rewrite fsetUA !(fsetUAC _ C) -(fsetUA _ C) fsetUid. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)