Goal:
    A : Type  B : Type  f : nat → A → B  l : list A
  ============================
   (∀ n : nat,
      mapi_rec f (List.rev l) n =
      List.rev (mapi_rec (λ (k : nat) (x : A), f (#|l| + n - S k) x) l 0))


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