xml - 限制检查 XML 节点中的元素对

Question

我有如下的 XML 文件

        <MyAttributes>
            <MyAttribute User = "CLASS_FROM_1" System = "CLASS1FROM"/>
            <MyAttribute User = "CODE_FROM_1" System = "CODE1FROM"/>
            <MyAttribute User = "CLASS_FROM_2" System = "CLASS2FROM"/>
            <MyAttribute User = "CODE_FROM_2" System = "CODE2FROM"/>
        </Attributes>

关键是我想检查是否有成对的MyAttribute元素（每对中的 CLASS 和 CODE），例如

        <MyAttribute User = "CLASS_FROM_1" System = "CLASS1FROM"/>
        <MyAttribute User = "CODE_FROM_1" System = "CODE1FROM"/>

在这种情况下：

    <MyAttributes>
        <MyAttribute User = "CLASS_FROM_1" System = "CLASS1FROM"/>
        <MyAttribute User = "CLASS_FROM_2" System = "CLASS2FROM"/>
        <MyAttribute User = "CODE_FROM_2" System = "CODE2FROM"/>
    </Attributes>

XML 不正确

是否可以在 XML Schema 1.0 中定义这样的约束（在 XSD 文件中进行验证）？也许改变 XML 的层次结构可以解决问题，但我不能改变 xml 结构。