code: def _next_file_gen(self, root):
      
      for e in root.getiterator(common._T_CONTENTS):
        st_ctime, size, etag, key = None, None, None, None
        for child in e.getiterator('*'):
          if child.tag == common._T_LAST_MODIFIED:
            st_ctime = common.dt_str_to_posix(child.text)
          elif child.tag == common._T_ETAG:
            etag = child.text
          elif child.tag == common._T_SIZE:
            size = child.text
          elif child.tag == common._T_KEY:
            key = child.text
        yield common.GCSFileStat(self._path + '/' + key,
                                 size, etag, st_ctime)
        e.clear()
      yield None
docs: Generator for next file element in the document.
  
      Args:
        root: root element of the XML tree.
  
      Yields:
        GCSFileStat for the next file.