Goal:
  
  A : Type
  f : nat → A
  m : nat
  t : A
  IHm : ∀ n : nat, nth_error (unfold m f) n = Some t → t = f n
  H : m < m + 1
  ============================
   (nth_error (unfold m f ++ [f m]) m = Some t → t = f m)


DEBUG:Going to execute:
rewrite nth_error_app_ge unfold_length // Nat.sub_diag /= //