XQuery代码完全由表达式组成，没有语句。
XQuery code consists entirely of expressions with no statements.
您可能已经注意到，清单2、清单3、清单4和清单5使用了完全相同的PHP代码(只有XPath表达式不同)。
You might have noticed that Listings 2, 3, 4, and 5 have the exact same PHP code-the only differences are the XPath expressions.
主题表达式包含表达式类型、完全限定主题名称和前缀映射。
A topic expression is defined in terms of expression type, fully qualified topic name, and prefix mapping.
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