SICP 练习2.28

(define (fringe items)
	(define (iter remainder-items result)
		(if (null? remainder-items)
		    result
		    (iter (cdr remainder-items)
		    	  (append result (if (pair? (car remainder-items))
		    	  			      (fringe (car remainder-items))
		    	  			      (cons (car remainder-items) '()))))))
	(iter items '()))