proposition: Lemma properC (T : finType) (A B : {set T}) : A \proper B = (~: B \proper ~: A).
proof: Proof. rewrite !properEneq setCS [~: _ == _]inj_eq 1?eq_sym //; exact/inv_inj/setCK. Qed.
imports: Coq(Setoid Morphisms) mathcomp(all_ssreflect) GraphTheory(edone)