Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  x : A
  l : list A
  IHl : ∀ n k : nat,
          k ≤ n
          → List.rev (mapi_rec f l (n - k)) =
            mapi_rec (λ (k0 : nat) (x : A), f (Nat.pred #|l| + n - k0) x)
              (List.rev l) k
  n : nat
  k : nat
  H : k ≤ n
  ============================
   (#|l| + (n - k) = Nat.pred (#|l| + 1) + n - k)


DEBUG:Going to execute:
intros; Zify.zify_pre_hook; zify_elim_let; zify_op