docs: 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