Ltac list_simpl := (repeat (progress (cbn; autorewrite with list))); cbn.Definition concat_map A B (f : A -> list B) x := concat (map f x).

(* Specifications: *)
Lemma L_ind_palindrome x A : L_ind P x A -> x = rev x <-> solution A.
Theorem PCP_CFG : PCP P <-> PALIN (f P).