Let's take a look at the L/N rule. We are told that L is 3 only if N is 5.
Only introduces a necessary condition, not a sufficient condition- so we would actually diagram this as follows.
If L3 -> N5 If N not 5 -> L not 3
In a more common sense analogy, I could say that I will go to the park only if it does not rain. In this case, we know if it rains I will not go to the park, and if I went to the park then it must not have rained. However, we know nothing about what must happen if it does not rain. I could go, but I do not have to.
