proposition: Lemma setUUr A B C : A :|: (B :|: C) = (A :|: B) :|: (A :|: C).
proof: Proof. by rewrite !(setUC A) setUUl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)