from, each time it starts on a new event boundary (this means that in
the old (5.3) version of the colorer, it also created a new co-routine
on each event boundary! (in other words, most of the reason one would
want co-routines here was bogus))
So, refactor the code to just always do this and eliminate a bunch of
set!'s and private fields in favor of just passing arguments like sane
code does. (We can't eliminate all of that, because we still do need
to be able to abort and thus all calls must be tail calls.)
original commit: 4ead534227fcbf8e90d0cc5e890f0a67fe6b05cb
4d5c904778