diff --git a/ex-4_01-xx.scm b/ex-4_01-xx.scm
index ce11716..c623605 100644
--- a/ex-4_01-xx.scm
+++ b/ex-4_01-xx.scm
@@ -214,5 +214,40 @@
 
 (display "\nex-4.9 - iteration-constructs\n")
 
+(define (repeat? exp) (tagged-list exp 'repeat))
+(define (repeat-count exp) (cadr exp))
+(define (repeat-body exp) (cddr exp))
+
+(define (repeat->combination exp)
+  (if (= (repeat-count exp) 0)
+    '()
+    (list
+      (sequence->exp (repeat-body exp))
+      (cons 'repeat
+            (cons (- (repeat-count exp) 1)
+                  (repeat-body exp))))))
+
+(display "repeat: ")
+(assert (repeat->combination '(repeat 10 (display "foo") (newline)))
+        '((begin (display "foo") (newline)) (repeat 9 (display "foo") (newline))))
+
+(define (while? exp) (tagged-list exp 'while))
+(define (while-predicate exp) (cadr exp))
+(define (while-body exp) (cddr exp))
+
+(define (while->combination exp)
+  (let ((body (while-body exp))
+        (predicate (while-predicate exp)))
+  (if (not (true? predicate))
+    '()
+    (list
+      (sequence->exp body)
+      (cons 'while
+            (cons predicate
+                  body))))))
+
+(display "while: ")
+(display (while->combination '(while (lambda () #t) (display "one"))))
+
 (display "\nex-4.10\n")