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


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