每个文件开头都会声明它属于哪一个包，每个文件也可以引入它所用到的包。

Each file begins with a simple declaration of what package it belongs to, and each file can import the packages it USES.

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XML Schema 1.1引入了一种称为类型替换(type alternatives)的新机制，允许模式作者在元素声明中指定类型替换。

XML schema 1.1 introduces a new mechanism called type alternatives that allow the schema author to specify type substitutions on an element declaration.

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import语句引入其他的命名空间(namespace)，但include语句却是将其他的声明引入到当前命名空间中。

An import statement brings in other namespaces. An include statement brings other declarations into the current namespace.

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