A fox USES its tail (or "brush") as a warm cover in cold weather and as a signal flag to communicate with other foxes.

狐狸用它的尾巴(或“毛刷”)在冷天时盖上保暖，同其他狐狸交流时亦可用来做信号旗。

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Then the rain came and the red flag went up to signal a halt.

当时，雨下了起来，红旗被升起，发出停止的信号。

《柯林斯英汉双解大词典》

The only thing you should normally do inside a signal handler is set a global flag that will be processed later.

在一个信号处理器内部，您通常应该做的一件事是，设置一个将在以后处理的全局标记。

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Generally, a bearish momentum divergence is a red flag warning signal, which often presages a pullback or correction.

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Their idealised version of this question was to ask how two signal chemicals, starting from opposite ends of an embryo, could result in a pattern that looked like the French flag (ie, three different coloured vertical stripes).

ECONOMIST: Genetic engineering