Goal:
  
  A : Type
  f : nat → A
  i : nat
  hf : ∀ i j : nat, f i = f j → i = j
  H : False
  ============================
   (i < 0)


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.