code: def get_opus_maximum_of(self, author_cts_urn):
          
          author = self.get_resource_by_urn(author_cts_urn)
          assert author is not None
          works = author.get_works()
  
          if len(works) > 1:
              for work in works:
                  if work.is_opus_maximum():
                      return work
          elif len(works) == 1:
              return works[0]
          else:
              return None
docs: Return the author's opux maximum (None otherwise).
  
          Given the CTS URN of an author, this method returns its opus maximum.
          If not available returns None.
  
          :param author_cts_urn: the author's CTS URN.
          :return: an instance of `surfext.HucitWork` or None