code: def get_pub_order(self, undefined=""):
          
          return _undefined_pattern(
              "".join(self.get_subfields("901", "f")),
              lambda x: x.strip() == "",
              undefined
          )
docs: Args:
              undefined (optional): Argument, which will be returned if the
                        `pub_order` record is not found.
  
          Returns:
              str: Information about order in which was the book published or \
                   `undefined` if `pub_order` is not found.