Once the expansion occurs on a newly created entity reference node, it never reoccurs.
新创建的实体引用节点进行扩展后，该扩展永远不会再次进行。
If there is no entity declaration that matches, an empty text node is attached as the only child of the entity reference node.
如果没有匹配的实体声明，则附加一个空的文本节点作为实体引用节点的唯一子级。
If there is no entity declaration that matches, an empty text node is attached as the only child of the entity reference node.
如果没有匹配的实体声明，则附加一个空的文本节点作为实体引用节点的唯一子级。
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