The key is the stylesheet processing instruction (PI) in the second line.

关键是第二行中的样式表处理指令（PI）。

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The second line is key, the stylesheet processing instruction (PI).

关键是第二行中的样式表处理指令(PI)。

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The key idea here is that when the first instruction runs, all of the subsequent instructions in that block are guaranteed to be executed without interruption.

这里的关键思想在于，当第一个指南运行的时候，该块中随后所有指南都一定会得到执行而且不会得到中断。

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