code: def luhn_calc(number, chars=DIGITS):
      '''
      Calculate the Luhn check digit for ``number``.
  
      :param number: string
      :param chars: string
  
      >>> luhn_calc('42')
      '2'
  
      '''
  
      checksum = luhn_checksum(str(number) + chars[0], chars)
      return chars[-checksumf luhn_calc(number, chars=DIGITS):
      '''
      Calculate the Luhn check digit for ``number``.
  
      :param number: string
      :param chars: string
  
      >>> luhn_calc('42')
      '2'
  
      '''
  
      checksum = luhn_checksum(str(number) + chars[0], chars)
      return chars[-checksum]
docs: Calculate the Luhn check digit for ``number``.
  
      :param number: string
      :param chars: string
  
      >>> luhn_calc('42')
      '2'