Calibre的主窗口会显示calibre库内有关现有文档的总结信息。
Calibre's main window displays summary information about the documents currently in your Calibre library.
这意味着命名空间可以出现在文档中的任何位置，但像编程变量一样有作用范围，命名空间只在相应的范围内有效。
This means that the namespaces can appear anywhere in the document, but have scope, like programming variables, and the namespaces are in effect for their appropriate scope.
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