解析后的XML文档是一个节点树，包含了文档节点及其后代节点。
The parsed XML document was a tree that contained the document node and its descendants.
这个节点可以有后代节点，这意味着它可以是整个 XML 文档的根（这种情况很常见）。
This single node can have descendants, which means it can be (and often is) the root of a full XML document.
这个方法告诉您所有想知道的信息：一个节点是否是另一个节点的祖先的后代，它是在前面还是后面呢，等等。
This method tells you all information you need to know: whether one node is descendant of an ancestor of the other, whether it is before or after, and so on.
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