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+= SICP exercise 3.19
+:categories: lisp programming-languages
+
+[source,scheme]
+----
+(define (cycle? l)
+  (define (rec l x)
+    (cond
+      ((null? x) false)
+      ((eq? l x) true)
+      (true (rec l (cdr x)))))
+  (rec l (cdr l)))
+----
+
+Sample interactive session:
+
+[source,scheme]
+----
+scheme@(guile-user)> (define true #t)
+scheme@(guile-user)> (define false #f)
+scheme@(guile-user)>
+(define (cycle? l)
+  (define (rec l x)
+    (cond
+      ((null? x) false)
+      ((eq? l x) true)
+      (true (rec l (cdr x)))))
+  (rec l (cdr l)))
+scheme@(guile-user)> (cycle? '(1 2 3))
+$9 = #f
+scheme@(guile-user)> (cycle? (make-cycle '(1 2 3)))
+$10 = #t
+----