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.
格里尔通过转换这些状态来实现向前和向后移动物体。
Switching between these states allows Grier to move objects back and forth.
另一方面，该机器可能想直接到达节点4，然后移到交互流的最后一个节点和最终状态。
On the other hand, the machine may want to directly target node 4 and then move to the final node and the final state of interaction flow.
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