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