docs: def split(string, callback=None, sep=None):
      """Split the string and execute the callback function on each part.
  
      >>> string = "1 2 3 4"
      >>> parts = split(string, int)
      >>> parts
      [1, 2, 3, 4]
  
      """
      if callback is not None:
          return [callback(i) for i in string.split(sep)]
      else:
          return string.split(sep)