diff --git a/memo.scm b/memo.scm
index a07136a7..66de1275 100644
--- a/memo.scm
+++ b/memo.scm
@@ -26,15 +26,15 @@
 (write (mfnc 1 1)) (newline)
 
 ; Original versions:
-(define (ack m n)
-  (cond ((= m 0) (+ n 1))
-        ((= n 0) (ack (- m 1) 1))
-        (else (ack (- m 1) (ack m (- n 1))))))
-(define (fib n)
-  (if (< n 2)
-      n
-      (+ (fib (- n 1))
-         (fib (- n 2)))))
+;(define (ack m n)
+;  (cond ((= m 0) (+ n 1))
+;        ((= n 0) (ack (- m 1) 1))
+;        (else (ack (- m 1) (ack m (- n 1))))))
+;(define (fib n)
+;  (if (< n 2)
+;      n
+;      (+ (fib (- n 1))
+;         (fib (- n 2)))))
 
 ; Fast versions:
 ;(define ack (memoize _ack))
@@ -50,6 +50,28 @@
 ;      (+ (fib (- n 1))
 ;         (fib (- n 2)))))
 
+;; New fast versions that do not introduce any new top-level definitions
+(define ack
+  ((lambda ()
+   (define (_ack m n)
+     (cond ((= m 0) (+ n 1))
+           ((= n 0) (__ack (- m 1) 1))
+           (else (__ack (- m 1) (__ack m (- n 1))))))
+   (define __ack (memoize _ack))
+   __ack
+  )))
+
+(define fib 
+  ((lambda ()
+    (define (_fib n)
+      (if (< n 2)
+          n
+          (+ (__fib (- n 1))
+             (__fib (- n 2)))))
+    (define __fib (memoize _fib))
+    __fib
+   )))
+
 (write (ack 3 12))
 (newline)
 (write (fib 40))