(define (return x) (tag 'done x))                 
(define (bounce thunk) (tag 'doing thunk))   
(define (call thunk) (thunk))    
(define (tag label thing) (cons label thing))
(define tag-of car)
(define tag-value cdr)

(define (pogo-stick thread)                                
  (cond ((eqv? 'done (tag-of thread))                      
          (tag-value thread))                              
        ((eqv? 'doing (tag-of thread))                     
          (pogo-stick (call (tag-value thread))))))          

(define (fact-iter n acc)
  (if (zero? n)
      (return acc)
      (bounce 
        (lambda ()
          (fact-iter
            (- n 1)
            (* acc n))))))

(define (mem? n lst)
  (cond ((null? lst) (return #f))
        ((= (car lst ) n) (return #t))
        (else (bounce
                (lambda ()
                  (mem? n (cdr lst)))))))

(define (fib n)
  (fib-iter n 0 0 1))

(define (fib-iter n i small large)
  (if (< i n)
      (bounce
        (lambda () 
          (fib-iter n (+ i 1) large (+ large small))))
      (return small)))


;  > (pogo-stick (fact-iter 5 1))
;  120
;  > (pogo-stick (mem? 5 (list 1 2 3 4 5 6)))
;  #t
;  > (pogo-stick (fib 8))
;  21