更有甚者，如果没有socket就绪而发生超时，那么该方法就会销毁调用者传入的参数，所以就算是什么事情都没有发生，调用者还是必须要在每一个循环中对这三个列表进行备份！
Worse, Select() clobbers the caller’s arguments if it times out and no socket is ready: the caller needs to make a copy of the three lists on each iteration even if nothing happens!
两个Alternative实例被用在不同的情况下等候警卫就绪，列表的顺序是隐式的优先级顺序。
The two Alternative instances are used in different circumstances to wait for guards to get ready, with the order of listing being the implicit priority orders.
可能的所有者可以在其工作列表中看到就绪任务。
The potential owner can see ready tasks on their work lists.
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