;starting-code

(define read-flatten-print
  (lambda ()
    (display "enter slist to flatten: ")
    (let ([slist (read)])
      (unless (eq? slist 'exit)
	(flatten-cps slist 
		     (lambda (val)
		       (pretty-print val)
		       (read-flatten-print)))))))

(define flatten-cps
  (lambda (ls k)
    (if (null? ls)
	(k ls)
	(flatten-cps (cdr ls)
	  (lambda (v) (if (list? (car ls))
			  (flatten-cps (car ls)
			      (lambda (u) (append-cps u v k)))
			  (k (cons (car ls) v))))))))

(define append-cps 
  (lambda (l1 l2 k)
    (if (null? l1)
	(k l2)
	(append-cps (cdr l1)
		    l2
		    (lambda (appended-cdr)
		      (k (cons (car l1)
			       appended-cdr)))))))