code: def get_part(self, undefined=""):
          
          return _undefined_pattern(
              "".join(self.get_subfields("245", "p")),
              lambda x: x.strip() == "",
              undefined
          )
docs: Args:
              undefined (optional): Argument, which will be returned if the
                        `part` record is not found.
  
          Returns:
              str: Which part of the book series is this record or `undefined` \
                   if `part` is not found.