code: def to_text(self, fields=None):
          
          if fields is None:
              return str(self) + '\n'
  
          try:
              result = ' '.join(str(self[x]) for x in fields) + '\n'
          except KeyError as ke:
              msg = 'Invalid field "{0}" found in fields= argument'
              msg = msg.format(str(ke))
              raise CDXException(msg)
  
          return result
docs: return plaintext CDX record (includes newline).
          if ``fields`` is ``None``, output will have all fields
          in the order they are stored.
  
          :param fields: list of field names to output.