Selects all the descendant nodes of the current node that match the selection criteria.
选取目前节点中，所有符合选取准则的子代节点。
2
Selects all the descendant nodes of the current node with the local name and namespace URI specified.
选取目前节点中，所有具有指定区域名称和命名空间uri的子代节点。
3
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.