diff --git a/Labs/04.rkt b/Labs/04.rkt
new file mode 100644
index 0000000..266ae0e
--- /dev/null
+++ b/Labs/04.rkt
@@ -0,0 +1,53 @@
+#lang plait
+(define-type MList
+  [MPair (n : Number) (tail : (Boxof MList))]
+  [Empty])
+
+(define (mlist lst)
+  (cond
+    [(empty? lst) (Empty)]
+    [else (MPair (first lst) (box (mlist (rest lst))))]))
+
+(test (mlist empty) (Empty))
+
+(test (mlist '(1)) (MPair 1 (box (Empty))))
+(test (mlist '(1 2)) (MPair 1 (box (MPair 2 (box (Empty))))))
+
+(define (take k mlst)
+  (cond
+    [(<= k 0) empty]
+    [else
+     (type-case MList mlst
+       [(Empty) empty]
+       [(MPair n tail-box)
+        (cons n (take (sub1 k) (unbox tail-box)))])]))
+
+(define big-mlist
+  (mlist (build-list 50 identity)))
+
+(test (take 10 big-mlist) '(0 1 2 3 4 5 6 7 8 9))
+
+(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)]))
+
+(define test-lst1 (mlist '(1 2 3)))
+(define test-lst2 (mlist '(4 5 6)))
+(set-last! test-lst1 test-lst2)
+(test (take 6 test-lst1) '(1 2 3 4 5 6))
+(test (take 3 test-lst2) '(4 5 6))
+(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))))
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