def get_resps(self, source_tree):
        """Returns a sorted list of all resps in `source_tree`, and the
        elements that bear @resp attributes.

        :param source_tree: XML tree of source document
        :type source_tree: `etree._ElementTree`
        :rtype: `tuple` of `lists`

        """
        resps = set()
        bearers = source_tree.xpath('//*[@resp]',
                                    namespaces=constants.NAMESPACES)
        for bearer in bearers:
            for resp in resp_splitter.split(bearer.get('resp')):
                if resp:
                    resps.add(tuple(resp.split('|', maxsplit=1)))
        return sorted(resps), bearers