proposition: Lemma setUA A B C : A :|: (B :|: C) = A :|: B :|: C.
proof: Proof. by apply/setP => x; rewrite !inE orbA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)