Goal:
  
  A : Type
  f : nat → A
  m : nat
  n : nat
  IHm : ∀ n : nat, n < m ↔ nth_error (unfold m f) n = Some (f n)
  Hn : n < S m
  n0 : n ≠ m
  ============================
   (nth_error (unfold m f) n = Some (f n))


DEBUG:Going to execute:
intros