Goal:
    A : Type  B : Type  P : list A → list A → A → A → B → Type
  ============================
   (All
      (λ z : B, All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z) [] [])
      [])


DEBUG:Going to execute:
fun x => lazymatch x with
                    | [] => idtac
                    | _ :: _ => idtac
                    end