Goal:
  
  A : Type
  f : nat → A
  m : nat
  n : nat
  t : A
  IHm : ∀ n : nat, nth_error (unfold m f) n = Some t → t = f n
  Hn : nth_error (unfold (S m) f) n = Some t
  ============================
   (t = f n)


DEBUG:Going to execute:
zify_op