diff --git a/Labs/04.rkt b/Labs/04.rkt
index 266ae0e..e574f4a 100644
--- a/Labs/04.rkt
+++ b/Labs/04.rkt
@@ -3,6 +3,7 @@
   [MPair (n : Number) (tail : (Boxof MList))]
   [Empty])
 
+; make
 (define (mlist lst)
   (cond
     [(empty? lst) (Empty)]
@@ -13,6 +14,7 @@
 (test (mlist '(1)) (MPair 1 (box (Empty))))
 (test (mlist '(1 2)) (MPair 1 (box (MPair 2 (box (Empty))))))
 
+; take
 (define (take k mlst)
   (cond
     [(<= k 0) empty]
@@ -22,18 +24,20 @@
        [(MPair n tail-box)
         (cons n (take (sub1 k) (unbox tail-box)))])]))
 
+; big
 (define big-mlist
   (mlist (build-list 50 identity)))
 
 (test (take 10 big-mlist) '(0 1 2 3 4 5 6 7 8 9))
 
+; set
 (define (set-last! lst1 lst2)
   (type-case MList lst1
     [(MPair n t)
      (type-case MList (unbox t)
        [(Empty)  (set-box! t lst2)]
-       [(MPair n t)  (set-last! (unbox t) lst2)])]
-     [(Empty) (void)]))
+       [else  (set-last! (unbox t) lst2)])]
+     [(Empty) (error 'empty "cannot set tail")]))
 
 (define test-lst1 (mlist '(1 2 3)))
 (define test-lst2 (mlist '(4 5 6)))
@@ -43,11 +47,17 @@
 (test (take 1000 test-lst1) '(1 2 3 4 5 6))
 (test/exn (set-last! (Empty) test-lst1) "cannot set tail")
 
-;(define small-cycle (cycle (mlist '(0 1 2))))
-;(define big-cycle (cycle big-mlist))
-;(test (cycle (Empty)) (Empty))
-;(test (take 0 big-cycle) empty)
-;(test (take 0 small-cycle) empty)
-;(test (take 5 big-cycle) '(0 1 2 3 4))
-;(test (take 5 small-cycle) '(0 1 2 0 1))
-;(test (take 107 big-cycle) (build-list 107 (lambda (n) (modulo n 50))))
\ No newline at end of file
+; cycle
+(define (cycle lst)
+  (type-case MList lst
+    [(MPair n t) (begin (set-last! lst lst) lst)]
+    [(Empty) (Empty)]))
+
+(define small-cycle (cycle (mlist '(0 1 2))))
+(define big-cycle (cycle big-mlist))
+(test (cycle (Empty)) (Empty))
+(test (take 0 big-cycle) empty)
+(test (take 0 small-cycle) empty)
+(test (take 5 big-cycle) '(0 1 2 3 4))
+(test (take 5 small-cycle) '(0 1 2 0 1))
+(test (take 107 big-cycle) (build-list 107 (lambda (n) (modulo n 50))))
\ No newline at end of file