code: def _add_childTnLst(self):
          
          self.remove(self.get_or_add_timing())
          timing = parse_xml(self._childTnLst_timing_xml())
          self._insert_timing(timing)
          return timing.xpath('./p:tnLst/p:par/p:cTn/p:childTnLst')[0]
docs: Add `./p:timing/p:tnLst/p:par/p:cTn/p:childTnLst` descendant.
  
          Any existing `p:timing` child element is ruthlessly removed and
          replaced.