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