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


DEBUG:Going to execute:
idtac