code: def _next_cTn_id(self):
          
          cTn_id_strs = self.xpath('/p:sld/p:timing//p:cTn/@id')
          ids = [int(id_str) for id_str in cTn_id_strs]
          return max(ids) + 1
docs: Return the next available unique ID (int) for p:cTn element.