Goal:
  
  A : Type
  f : nat → A
  m : nat
  n : nat
  IHm : ∀ n : nat, n < m ↔ nth_error (unfold m f) n = Some (f n)
  n0 : BinInt.Z.of_nat n ≠ BinInt.Z.of_nat m
  Hn : BinInt.Z.lt (BinInt.Z.of_nat n)
         (BinInt.Z.add (BinInt.Z.of_nat m) (BinNums.Zpos BinNums.xH))
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat m)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  ============================
   (nth_error (unfold m f) n = Some (f n))


DEBUG:Going to execute:
idtac