The nested structure gives a sequence and hierarchy of topics within a topic collection.

嵌套的结构在主题集合中给出一个主题序列和层次结构。

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That is, you can treat the set of topics as a collective content object, using the root topic to represent the collection as a whole for navigation and cross references.

也就是说，您可以将主题集看作是一个集合性的内容对象，使用根主题将集合表示成导航和交叉引用的一个整体。

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To restrict the contents of the collection, requiring specific topic types or roles at specified positions within the collection.

限制集合的内容，要求在集合中的特定位置出现特定的主题类型或者角色。

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Visitors are encouraged to peruse the book collection, which ranges in topic from sex, monsters and medicine to the occult, folklore and natural history.

BBC: New York City��s most morbid museum

You may want to start by reading a collection of papers prepared for PCAST (pdf) on the topic.

WHITEHOUSE: PCAST Launches Policy Forum on the Future of U.S. Advanced Manufacturing | The White House