Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  x : A
  l : list A
  IHl : ∀ 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)
  n : nat
  ============================
   (f n x = f (#|l| + 1 + n - S (#|l| + 0)) x)


DEBUG:Going to execute:
f_equal
DEBUG LTAC Evaluated term: length_app
DEBUG LTAC Evaluated term: length_app