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:
<coq-core.plugins.ltac::contradiction@0> $1
Level 0: Backtrace:

No primitive equality found.