not if then element 反若则元件
If "phone" was an attribute of the employee element, then it can exist only once per employee, and we could not add child elements too it, which may hinder schema evolution over time.
如果“phone”是雇员元素的一个属性，那么对于每个雇员，它只能出现一次，我们也不能为之添加子元素，这将妨碍模式随时间而变化。
If the to element is not present inside the path header entry, then the ultimate destination is designated by the last via element.
如果to元素没出现在path头条目中，则由最后一个via元素指明最终目的地。
If the element does not have an href attribute, then you can transform it to a element instead of a element.
如果元素没有href属性，可以将其转换成元素而不是元素。
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