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