Apply changes from John Cowan.

ephemerons-racket.scm

@@ -0,0 +1,21 @@
+(define %make-ephemeron make-ephemeron)
+
+;; ephemeron? is already defined correctly
+
+(define (make-ephemeron key datum)
+  (%make-ephemeron key (cons key datum)))
+
+(define (ephemeron-broken? ephemeron)
+  (not (ephemeron-value ephemeron)))
+
+(define (ephemeron-key ephemeron)
+  (let ((value (ephemeron-value ephemeron)))
+    (if (eq? value #f)
+      #f
+      (car value))))
+
+(define (ephemeron-datum ephemeron)
+  (let ((value (ephemeron-value ephemeron)))
+    (if (eq? value #f)
+      #f
+      (cdr value))))

ephemerons-trivial.scm

@@ -0,0 +1,10 @@
+(define-record <ephemeron>
+  (%make-ephemeron key datum broken?)
+  ephemeron?
+  (key ephemeron-key)
+  (datum ephemeron-datum)
+  (broken? ephemeron-broken?))
+
+(define (make-ephemeron key datum) (%make-ephemeron key datum #f))
+
+(define (reference-barrier key) #t)