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+---
+
+title: SICP exercise 3.19
+
+date: 2021-09-02
+
+layout: post
+
+lang: en
+
+ref: sicp-exercise-3-19
+
+---
+
+```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:
+
+```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
+```