Goal:
  
  A : Type
  f : nat → A
  n : nat
  i : nat
  hf : ∀ i j : nat, f i = f j → i = j
  IHn : In (f i) (unfold n f) → i < n
  H : f n = f i ∨ False
  ============================
   (i < S n)


DEBUG:Going to execute:
destruct H