(define rec
 (lambda (n stop)
  (display n) (newline)
  (if (= n 0) (stop 0)
   (begin
     (rec (- n 1) stop)
     (display n) (newline)))))
(rec 6 (lambda (x) '()))
(call-with-current-continuation
 (lambda (t) (rec 6 t)))