Complete exercise 3.18

3_16.rkt

@@ -28,6 +28,16 @@
               1))))
   (count l '()))
           
+(define (has-cycle? l)
+  (define (cycle-iter l seen-pairs)
+    (cond ((not (pair? l)) #f)
+          ((memq (cdr l) seen-pairs) #t)
+          (else
+           (cycle-iter (cdr l)
+                       (cons l seen-pairs)))))
+  (cycle-iter l '()))
+  
+
 ;; Both give 3
 (define l1 '(a b c))