SICP 联系2.28 实现fringe

(define (fringe x)  (cond ((null? (car x)) #f)    ((pair? (car x)) (fringe (car x)))    (else (out-elem (car x))))  (cond ((null? (cdr x)) #f)    ((pair? (cdr x)) (fringe (cdr x)))    (else (out-elem (cdr x)))))(define (out-elem x)  (display x)  (display " "))(define x (list (list 1 2) (list 3 4)))(newline)(fringe x)(newline)(fringe (list x x))