Fixed another oversight.
Update time log.
checkpoint
bugfix
checkpoint-of-bogus-find-prefixes
checkpoint.  first plausible prefix map builder.
Rewrote `build-prefixes` to be a little bit clearer.

Its still not really clean though.

I should either separate out the `dropped...` construction (or figure
out how to eliminate it entirely; I put it in to accommodate
left-recursion while still assigning positive weight to symbols in
`prodseq-equal-upto`).
.