请大家帮我解释一个关于环绕中节点的概念，一下引一小段文献
At first glance the molecular knot seems to be a figure-eight (41) knot rather than trefoil knot. However, the over/under (O/U) sequence of the crossings in the typical figure-eight is alternate U–O–U–O in the direction of the arrow, whereas that in 1 is U–O–O–U.
请问over/under (O/U) sequence是什么意思，如何判断O-U的顺序
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