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

(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) (apply-continuation unchanged)]
       [(symbol? (car slist))
        (if (eqv? (car slist) old)
            (apply-continuation changed (cons new (cdr slist)))
            (loop (cdr slist)
                  (lambda (substituted-cdr)
                    (apply-continuation changed (cons (car slist)
						      substituted-cdr)))
                  unchanged))]
       [else
        (loop (car slist)
              (lambda (substituted-car)
                (apply-continuation changed (cons substituted-car
						  (cdr slist))))
              (lambda ()
                (loop (cdr slist)
                      (lambda (substituted-cdr)
                        (apply-continuation changed (cons (car slist)
							  substituted-cdr)))
                      unchanged))
	      )]))))

(substitute-leftmost 'x 's '((c a (b a) () ( () d s))))