docs: 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