当这个元素等于 true 时，它将一个引用分配给 Machine 对象，而后者将在规则的结果块被用到。

Whenever this element evaluates to true, it assigns a reference to the Machine object that is then used in the consequence part of the rule.

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ActionScript在使用一个包(库)之前，先要进行引用，所以例中脚本块的开头有一行“ import ”语句。

ActionScript also requires a reference to other packages (libraries) in order to use them; hence the "import" statement at the top of the Script block.

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最后，字符串类型是引用类型，因此每个字符串实例都引用一个附加的内存块，以存放实际的字符串数据。

Finally, the string type is a reference type, so every string instance refers to an additional block of memory to hold the actual string data.

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