code: def luhn_checksum(num: str) -> str:
      
      check = 0
      for i, s in enumerate(reversed(num)):
          sx = int(s)
          sx = sx * 2 if i % 2 == 0 else sx
          sx = sx - 9 if sx > 9 else sx
          check += sx
      return str(check * 9 % 10)
docs: Calculate a checksum for num using the Luhn algorithm.
  
      :param num: The number to calculate a checksum for as a string.
      :return: Checksum for number.