signal constraint

例句：

1. 1. The traffic light is signaling the driver to stop.
   交通灯正在指示司机停车。
2. 2. The new policy will signal a change in our approach to environmental protection.
   新政策将标志着我们环境保护方法的改变。
3. 3.The sudden drop in stock prices signaled an impending recession.
   股票价格的突然下跌预示着即将到来的经济衰退。