proposition: Lemma dec_enc t : dec (exists n, t = nat_enc n).
proof: Proof. destruct (nat_unenc t) eqn:H. - left. exists n. now eapply unenc_correct2 in H. - right. intros [n A]. rewrite A in H. rewrite unenc_correct in H. inv H. Qed.
imports: Undecidability.Shared.Libs.PSL.Numbers Nat Undecidability.L(Datatypes.LBool Functions.EqBool Datatypes.LProd)