Returns True if all titleInfo subfields are not set or
        empty; returns False if any of the fields are not empty.

def is_empty(self):
        '''Returns True if all titleInfo subfields are not set or
        empty; returns False if any of the fields are not empty.'''
        return not bool(self.title or self.subtitle or self.part_number \
                        or self.part_name or self.non_sort or self.type)