code: def isbn(self):
          
          isbns = listify(chained_get(self._head, ['source', 'isbn'], []))
          if len(isbns) == 0:
              return None
          else:
              return tuple((i['$'] for i in isbns))
docs: ISBNs belonging to publicationName as tuple of variying length,
          (e.g. ISBN-10 or ISBN-13).