Get the subject of the object

        :param lang: Lang to retrieve
        :return: Subject string representation
        :rtype: Literal

def get_subject(self, lang=None):
        """ Get the subject of the object

        :param lang: Lang to retrieve
        :return: Subject string representation
        :rtype: Literal
        """
        return self.metadata.get_single(key=DC.subject, lang=lang)