Goal:
    A : Type  B : Type  f : nat → A → B  l : list A
  ============================
   (∀ 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)


DEBUG:Going to execute:
induction l using rev_ind; simpl