docs: def _tidy(self, work, file_path):
          """Transforms the file at `file_path` into simpler XML and returns
          that.
  
          """
          output_file = os.path.join(self._output_dir, work)
          self._logger.info('Tidying file {} into {}'.format(
              file_path, output_file))
          try:
              tei_doc = etree.parse(file_path)
          except etree.XMLSyntaxError as err:
              self._logger.error('XML file "{}" is invalid: {}'.format(
                  file_path, err))
              raise
          return self.transform(tei_doc).getroot()