Date: Sat, 27 Aug 2016 15:40:16 +0000
   From: Marc Nieper-Wißkirchen <xxxxxx@nieper-wisskirchen.de>

   > 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.

   Is it a hard fact that it may be non-trivial to implement
   `reference-barrier' for a given proper implementation of ephemerons?

Here's an implementation of reference-barrier that will almost
certainly have the desired immediate effect in any implementation,
though it may leak space.

(define last-reference 0)
(define (reference-barrier x)
  (let ((y last-reference))
    (set! last-reference x)
    y))

I have a hard time believing that there are any Scheme systems out
there in which it is actually difficult to implement reference-barrier
correctly.  It should be reliably included.

   On the other hand, how do you want to code safely without
   `reference-barrier'?

Can't.