Fix sequence rewrite description by eliminating nullables first
Fix sequence rewrite description by eliminating nullables first

Fix a bug which caused nullable terminated or liberal sequences
such as (item, 0, n, sep, 'terminator') to get rewritten into rules
that match a single trailing `sep`.

jeffreykegler/kollos,jeffreykegler/kollos,jeffreykegler/kollos,jeffreykegler/kollos,jeffreykegler/kollos