code: def get_format(self, undefined=""):
          
          return _undefined_pattern(
              "".join(self.get_subfields("300", "c")),
              lambda x: x.strip() == "",
              undefined
          )
docs: Args:
              undefined (optional): Argument, which will be returned if the
                        `format` record is not found.
  
          Returns:
              str: Dimensions of the book ('``23 cm``' for example) or 
                  `undefined` if `format` is not found.