induction 1; intuition.
repeat rewrite app_nil_l.
econstructor.
intuition.
econstructor.
repeat rewrite <- app_comm_cons.
econstructor; intuition.
Qed.

(* Specifications: *)
Theorem list_pred_impl'
: forall (A B : Set) (lsa : list A) (lsb : list B) (P1 : A -> B -> Prop),
list_pred P1 lsa lsb ->
forall P2 : A -> B -> Prop,
(forall (a : A) (b : B), In a lsa -> In b lsb -> P1 a b -> P2 a b) -> list_pred P2 lsa lsb.