PPS The following program for Chez Scheme demonstrates the issue (not
that I dropped the argument to G, which I also dropped in my text
above (but forgot to remove in the code example):

**
(import (scheme))

(define eph #f)

(define (f v)
  (set! eph (ephemeron-cons v #t)))

(define (g)
  (car eph))

(define (g/collect)
  (collect)                             ;simulate GC
  (car eph))

(define foo
  (lambda (f g)
    (let ((v (vector 1 2)))
      (f v)
      (let ((w (g)))
        (boolean=? (vector? v) (vector? w))))))

(assert (foo f g))
(assert (foo f g/collect))              ;fails
**