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1·Evolve the XML schema (upward compatibility).

更新XML schema(向上兼容性)。

2·This upward compatibility is useful when you want to combine sets of documentation from two sources, each of which has specialized differently.

如果需要将两种不同来源、分别具有不同专门化的文档组合起来，那么这种向上兼容性非常有用。

3·A concern with evolving XML schema with upward compatibility is that XML data cannot be validated with the existence of newly added elements and attributes.

使用向上兼容性更新XML schema时需要考虑的一个问题是：XML数据具有新添加的元素和属性时不能通过验证。

4·I've already mentioned the problem about evolving an XML schema with upward compatibility, where the existence of newly added elements and attributes cannot be checked.

我已经提到过具有向上兼容性的XML schema更新的问题，即不能检查新添加的元素和属性是否存在。