Switch to "neither" from "not".

[DanielRosenwasser]

This is a nit, and I will close it if it goes on for more than 2-3 comments.

This just reads better to me when a nor is matched with a neither, and most places in the spec that use nor with a group of values uses neither.

Alternatively, the only other place in the spec where we seem to negate values of null or undefined with "not" uses the word "or" instead of "nor", and that would probably sound best to me.

[jridgewell]

with "not" uses the word "or" instead of "nor", and that would probably sound best to me.

"nor" should always be used when describing a negative union.