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))