(define append
  (lambda (L1 L2 k)
    (if (null? L1)
	(k L2)
	(append (cdr L1) L2 (lambda (hole) (k (cons (car L1) hole)))))))

(define up
  (lambda (lst k)
    (if (null? lst)
	(k '())
	(if (pair? (car lst))
	    (up (cdr lst) (lambda (hole) (append (car lst) hole k)))
	    (up (cdr lst) (lambda (hole) (k (cons (car lst) hole))))))))