def get_witnesses(self, source_tree):
        """Returns a sorted list of all witnesses of readings in
        `source_tree`, and the elements that bear @wit attributes.

        :param source_tree: XML tree of source document
        :type source_tree: `etree._ElementTree`
        :rtype: `tuple` of `list`\s

        """
        witnesses = set()
        bearers = source_tree.xpath('//tei:app/tei:*[@wit]',
                                    namespaces=constants.NAMESPACES)
        for bearer in bearers:
            for witness in witnesses_splitter.split(bearer.get('wit')):
                if witness:
                    witnesses.add(witness)
        return sorted(witnesses), bearers