; Day 24  continutations examples.

(define apply-continuation
  (lambda (k .  v)
    (apply k v)))

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

(define fact-cps
  (trace-lambda fact-cps (n k)
    (if (zero? n)
	(apply-continuation k 1)
	(fact-cps (- n 1)
		  (lambda (v)
		    (apply-continuation k (* n v)))))))

(define list-copy-cps
  (lambda (L k)
    (if (null? L)
	(apply-continuation k '())
	(list-copy-cps (cdr L)
		       (lambda (v)
			 (apply-continuation k (cons (car L) v))))))) 


(define memq-cps
  (lambda (sym ls k)
    (cond [(null? ls) (apply-continuation k #f)]
	  [(eq? sym (car ls))
	   (apply-continuation k #t)]
	  [else
	   (memq-cps sym (cdr ls) k)])))
		     

(define intersection  ; convert this to CPS
  (lambda (los1 los2)
    (cond
     [(null? los1) '()]
     [(memq (car los1) los2)
      (cons (car los1)
            (intersection (cdr los1) los2))]
     [else (intersection (cdr los1) los2)])))


(define free-vars ; convert to CPS
  (lambda (exp)
    (cond [(symbol? exp) (list exp)]
	  [(eq? (1st exp) 'lambda)       
	   (remove (car (2nd exp)) 
		   (free-vars (3rd exp)))]      
	  [else (union (free-vars (1st exp))
		       (free-vars (2nd exp)))])))





(define substitute-leftmost
  (lambda (new old slist)
    (subst-left-cps
      new
      old
      slist
      (lambda (v) v)     ; succeed continuation
      (lambda () slist)  ; fail continuation
     )))

(define subst-left-cps
  (lambda (new old slist changed unchanged)
    (let loop ([slist slist] 
               [changed changed] 
               [unchanged unchanged])
      (cond
       [(null? slist) (unchanged)]  
.