It's optional because it has to do something that in a non-trivial implementation of ephemerons, such as Racket's, can't always be done.  If you need reference barriers, your code willl not be portable to all R7RS-large implementations, which is a pity, but there it is.

We'll take up the question of new feature flags near the end of the process.  `reference-barriers` is a good candidate for one.

-- 

John Cowan          http://www.ccil.org/~cowan        xxxxxx@ccil.org

We want more school houses and less jails; more books and less arsenals;

more learning and less vice; more constant work and less crime; more

leisure and less greed; more justice and less revenge; in fact, more of

the opportunities to cultivate our better natures.  --Samuel Gompers