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