I've found two bugs in SubX libraries:
1. next-word had an out-of-bounds read
2. next-word was skipping comments, because that's what I need during bootstrapping.
I've created a new variant called next-raw-word that doesn't skip comments.
These really need better names.
We're now at the point where 4b.mu has the right structure and returns
identical result to 4a.mu.
Kartik Agaram