The state flower is the red clover and the tree is the sugar maple.
它的州花是红花苜宿，而它的州树是糖枫树。
If the tree node is expandable, it will get the state of expanded.
如果树节点是可展开的，则将会获得“expanded”状态。
At the end of tree-visitor, both the final state and the final tree are returned.
tree- visitor结束时将同时返回最终状态和最终树。
一个国家，好比一棵树。
一个国家，好比一棵树。
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