<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_urn(self):
 """ Assumes that each HucitAuthor has only one CTS URN. """

        # TODO: check type
        try:
            type_ctsurn = self.session.get_resource(BASE_URI_TYPES % "CTS_URN"
                                                    , self.session.get_class(surf.ns.ECRM['E55_Type']))
            urn = [CTS_URN(urnstring.rdfs_label.one)
                                for urnstring in self.ecrm_P1_is_identified_by
                                                            if urnstring.uri == surf.ns.ECRM['E42_Identifier']
                                                            and urnstring.ecrm_P2_has_type.first == type_ctsurn][0]
            return urn
        except Exception as e:
            return None