docs: def difference(array, k=1):
      """Calculate l[n] - l[n-k]
      """
      if (len(array) - k) < 1:
          raise ValueError()
      if k < 0:
          raise ValueError("k has to be greater or equal than zero!")
      elif k == 0:
          return [i - i for i in array]
      else:
          return [j - i for i, j in zip(array[:-k], array[k:])]