伪列是具有预先定义的名称的标识符，其中包含关于分级查询的信息；它的用途类似于表中的任何列。
A pseudo column is an identifier with a predefined name that holds information about the hierarchical query and that can be used like any column in a table.
如果uri以url的形式出现，那么甚至不需要引用一个实际的Web页面；它可以是一个“伪”URL，只是作为一个标识符。
If the URI takes the form of a URL, it doesn't even need to reference a real Web page; it can be a "fake" URL that simply serves as an identifier.
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