(* Lemma red_turing_compl X Y (Q : Y -> Prop) (P : X -> Prop) : *)
(*   Q ⪯ P -> compl Q ⪯T compl P. *)
(* Proof. *)
(*   intros [f] [d]. econstructor. intros x. *)
(*   destruct (d (f x)). *)
(*   + left. firstorder. *)
(*   + right. firstorder. *)
(* Qed. *)

Lemma undec_PCP X (P : X -> Prop) :
undec P <-> (PCP ⪯T P).