Return the next available unique ID (int) for p:cTn element.

def _next_cTn_id(self):
        """Return the next available unique ID (int) for p:cTn element."""
        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