chibi-scheme/lib/srfi/124.sld

(define-library (srfi 124)
  (export make-ephemeron ephemeron? ephemeron-broken?
          ephemeron-key ephemeron-datum reference-barrier)
  (import (rename (chibi weak) (ephemeron-value ephemeron-datum))
          (only (scheme base) define if))
  (begin
    (define (reference-barrier k) (if #f #f))))