Goal:
  
  A : Type
  f : nat → A
  n : nat
  Hn : BinInt.Z.lt (BinInt.Z.of_nat n) BinNums.Z0
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  ============================
   (nth_error (unfold 0 f) n = Some (f n))


DEBUG:Going to execute:
idtac