Hi Min —
Exactly one plane after N and before R means that there is exactly one other element that goes in between N and R, going after N and before R — since we don’t know what that one element is, we can notate the rule like so:
N _ R
The final rule gives us SR or RS.
If we combine those rules, when S goes immediately before R, S must be the one element that fits in between N and R, like so: NSR.
When S goes immediately after R, we can notate the combination of rules like so: N _ RS
Hope that clears it up and let me know if you have any follow-up q’s — Mike