By combining the two kinds of entities, the shell DTD allows the specialized content elements to be used in the same contexts as the base element.

通过结合这两种实体，外壳DTD允许将专门化的内容元素作为基本元素在相同的上下文中使用。

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When used through the "contribution" pattern, an element can add new content to a base element without directly modifying any of the existing properties.

当在“贡献”模型中被使用的时候，一个元素能够将新的内容添加到一个基本元素之中，而无需直接修改现任何一个已存在的性质。

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The third element of the base class is used to choose which specific template specialization to use. The value for the third element is provided by

基类的第三个元素用于选择要使用哪种具体的模板特殊化。

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