proposition: Lemma equipollent_Deq : forall (A B:Type), Deq A -> equipollent A B -> Deq B.
proof: Proof. unfold Deq, equipollent, bijection. introv Ha Heq. intros. exrepnd. pose proof (Ha (g x) (g y)) as Hd. dorn Hd; [left| right]. - apply left_id_injection in Heq1. repnud Heq1. auto. - introv Heq. apply Hd. f_equal; auto. Qed.
imports: Coq.Init.Notations tactics Peano Basics Bool Arith Arith.EqNat Omega