Remember that the AST classes aren't visible outside of this package at all and that clients will (eventually) simply pass in a String representation of the expression they want evaluated.

要记住 AST 类在包外是不可见的， 还有就是客户机（最终）只会传入它们想求值的表达式的一个字符串表示。

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The pseudo column LEVEL is used in a string to make the hierarchy visible.

可在字符串中使用伪列LEVEL，使分层结构变得可视。

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The Label field contains the string that will be visible to the user when the analysis configuration dialog is visible.

Label 区域包含了，当分析配置目录可见时，对用户可见的字符串。

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