End Forall2.

(* Specifications: *)
Lemma Forall2_impl (A B : Type) (R1 R2 : A -> B -> Prop) : (forall a b, R1 a b -> R2 a b) ->
forall l1 l2, Forall2 R1 l1 l2 -> Forall2 R2 l1 l2.
Lemma Forall2_flip (A B : Type) (R : A -> B -> Prop) l1 l2 :
Forall2 R l1 l2 -> Forall2 (fun b a => R a b) l2 l1.