docs: def handle_req(msg):
      """Process a req message.
  
      This will return the value if it exists. If no value exists,
      nothing is returned.
      """
      if not msg.gateway.is_sensor(msg.node_id, msg.child_id):
          return None
      value = msg.gateway.sensors[msg.node_id].children[
          msg.child_id].values.get(msg.sub_type)
      if value is not None:
          return msg.copy(
              type=msg.gateway.const.MessageType.set, payload=value)
      return None