Reason about the data in the expression tree, and do custom processing over it.
推断表达式树内数据，并对它做自定义处理。
It includes some features of a general-purpose expression language and is designed to be a little language that can be used for application-neutral processing within XML systems.
它具备通用表达式语言的若干特性，旨在提供一种轻量型语言，可用于XML系统内与应用程序无关的处理。
XPath (covered earlier) is used for accessing the source document and general expression processing.
使用XPath（ 前文 已述）访问原文档和生成表达处理。
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