2.28

练习 2.28 写一个过程fringe，它以一棵树（表示为表）为参数，返回一个表，表中的元素是这棵树的所有树叶，按照从左到右的顺序。例如：

(define x (list (list 1 2) (list 3 4)))
(fringe x)
(1 2 3 4)

(fringe (list x x))
(1 2 3 4 1 2 3 4)

(define x (list (list 1 2) (list 3 4)))
x

(define (fringe x)
  (if (null? x) '()
    (if (and (pair? (car x)) (not (pair? (car (car x)))))
        (append (car x) (car (cdr x)))
        (append (fringe (car x)) (fringe (cdr x)))
    )
  )
)

(fringe x)

(fringe (list x x))