diff --git a/contrib/20.r7rs/scheme/lazy.scm b/contrib/20.r7rs/scheme/lazy.scm
index c2118797..f429dcd0 100644
--- a/contrib/20.r7rs/scheme/lazy.scm
+++ b/contrib/20.r7rs/scheme/lazy.scm
@@ -1,60 +1,41 @@
 ;;; Appendix A. Standard Libraries Lazy
 
 (define-library (scheme lazy)
-  (import (scheme base)
-	  (picrin macro))
+  (import (scheme base))
+
+  ;; type 'a <promise> = cached of 'a | chained of 'a promise | pending of () -> 'a promise
 
   (define-record-type <promise>
-      (make-promise% done value)
-      promise?%
-    (done  promise-done?% set-promise-done!%)
-    (value promise-value% set-promise-value!%))
+      (promise state value)
+      promise?
+    (state promise-state set-promise-state!)
+    (value promise-value set-promise-value!))
 
-  (define (box x) (list x))
-  (define box? list?)
-  (define unbox car)
-  (define set-box! set-car!)
+  (define (make-promise obj)
+    (if (promise? obj)
+	obj
+	(promise 'cached obj)))
 
-  (define (promise? x)
-    (promise?% (unbox x)))
-  (define (promise-done? x)
-    (promise-done?% (unbox x)))
-  (define (set-promise-done! boxed x)
-    (set-promise-done!% (unbox boxed) x))
-  (define (promise-value x)
-    (promise-value% (unbox x)))
-  (define (set-promise-value! boxed x)
-    (set-promise-value!% (unbox boxed) x))
-
-  (define (make-promise%% done value)
-    (box (make-promise% done value)))
-  
   (define-syntax delay-force
     (syntax-rules ()
       ((_ expr)
-       (make-promise%% #f (lambda () expr)))))
+       (promise (string->symbol "pending") (lambda () expr)))))
 
   (define-syntax delay
     (syntax-rules ()
       ((_ expr)
-       (delay-force (make-promise%% #t expr)))))
+       (delay-force (make-promise expr)))))
 
-  (define (force promise)
-    (if (promise-done? promise)
-	(promise-value promise)
-	(let ((new-promise ((promise-value promise))))
-          (if (promise-done? promise)
-              (promise-value promise)
-              (begin
-                (set-promise-done!  promise (promise-done? new-promise))
-                (set-promise-value! promise (promise-value new-promise))
-                (set-box! new-promise (unbox promise))
-                (force promise))))))
-
-  (define (make-promise obj)
-    (if (and (box? obj) (promise? obj))
-	obj
-	(make-promise%% #t obj)))
+  (define (force p)
+    (let ((v (promise-value p)))
+      (case (promise-state p)
+        ((cached)  v)
+        ((chained) (force v))
+        ((pending) (let ((q (v)))
+                     (when (eq? 'pending (promise-state p))
+                       (set-promise-state! p 'chained)
+                       (set-promise-value! p q))
+                     (force p))))))
 
   (export delay-force
           delay