第四个类型替换将检查 type 属性是否存在。

The fourth type alternative checks whether the type attribute exists.

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为了克服xsi： type的缺陷，可以使用类型替换机制。

To address the shortcomings of xsi: type, you can use the type alternative mechanism.

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如果元素声明具有类型替换，那么将按照在模式中指定的顺序进行计算。

If an element declaration has type alternatives, they are evaluated in the order that they were specified in the schema.

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