类别也是可浏览的，每个类别只具有一个父类别。
Categories are also browsable, with each having only one parent category.
平原对象定位使得在父类别预计的地方使用派生的任何类。
Plain object orientation makes it possible to use any derived classes in places where the parent class is expected.
由于文档和其他类别可以继承父类别的标记，从而产生一个完整的层次结构。
Both documents and other categories can be tagged with a parent category, resulting in a complete hierarchy.
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