Add `./p:timing/p:tnLst/p:par/p:cTn/p:childTnLst` descendant.

        Any existing `p:timing` child element is ruthlessly removed and
        replaced.

def _add_childTnLst(self):
        """Add `./p:timing/p:tnLst/p:par/p:cTn/p:childTnLst` descendant.

        Any existing `p:timing` child element is ruthlessly removed and
        replaced.
        """
        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]