So, using our simple valence bond theory, what we would expect is that we want to pair up any unpaired electrons in methane with unpaired electrons from hydrogen and form bonds.

利用简单的价电子成键理论，我们预计,要把所有甲烷中没有配对的电子,和氢原子中没有配对的电子配对来形成键。

麻省理工公开课 - 化学原理课程节选

If it isn't, then I'm going to go back around the loop, and notice what happens. If I get all the way around the loop, that is, all the way through that FOR loop and I never find a path that takes me through here, then the last thing I'm going to do is return a pair or a tuple with a special simple number none twice.

如果没有,我会回到循环看看发生了什么,如果我运行了整个循环,也就是整个for循环,我仍没有找到符合条件的路径,那么我最后做的事情就是返回一对,或者一个数组的两次none值。

麻省理工公开课 - 计算机科学及编程导论课程节选