Floyd's Cycle Detection Algorithm

gistfile1.txt

(define (contains-cycle? lst)
  (let inner ((a (nil-safe-cdr lst))
              (b (nil-safe-cdr (nil-safe-cdr lst))))
     (cond ((or (not (pair? a))
                (not (pair? b))) #f)
           ((or (eq? a b)
                (eq? a (nil-safe-cdr b))) #t)
           (else (inner (nil-safe-cdr a) (nil-safe-cdr (nil-safe-cdr b)))))))