1 |(defun cycle-length (n & (len 1) (top 0))
  |cond ((= n 1) (values len top))
  |     ((evenp n) (cycle-length (ash n -1) (1+ len) (max top n)))
  |     (t (let ((next (1+ (* 3 n))))
5 |          (cycle-length next (1+ len) (max top next))))))