def get_opus_maximum_of(self, author_cts_urn):
        """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

        """
        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