为了说明这个作法，图5显示一个虚构的迭代器的状态，它能够向后移动一、二、三、四个位置，但不能移动更多位置，因为这要求访问前面访问的叶节点。

To illustrate, Figure 5 shows the state of a hypothetical iterator that can move back one, two, three, or four places, but no more, because that would require accessing the previously visited leaf.

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