code: def add_delayed(self, parent, rule_function, child_names):
          
          if parent in self._aggregates or parent in self._incomplete_aggregates:
              raise ValueError("Sensor %r already has an aggregate rule"
                               " associated" % parent)
          reg = self._registered_sensors
          names = set(name for name in child_names if name not in reg)
          sensors = set(reg[name] for name in child_names if name in reg)
          if names:
              self._incomplete_aggregates[parent] = (rule_function, names,
                                                     sensors)
          else:
              self.add(parent, rule_function, sensors)
docs: Create an aggregation rule before child sensors are present.
  
          Parameters
          ----------
          parent : :class:`katcp.Sensor` object
              The aggregate sensor.
          rule_function : f(parent, children)
              Function to update the parent sensor value.
          child_names : sequence of str
              The names of the sensors the aggregate sensor depends on. These
              sensor must be registered using :meth:`register_sensor` to become
              active.