2.35

练习 2.35 将2.2.2节的count-leaves重新定义为一个累积：

(define (count-leaves t)
    (accumulate <??> <??> (map <??> <??>)))

首先，定义 accumulate

(define (accumulate op initial sequence)
    (if (null? sequence)
        initial
        (op (car sequence)
            (accumulate op initial (cdr sequence))) 
    )
)

然后，再定义 enumerate-tree

(define (enumerate-tree tree)
    (cond ((null? tree) '())
        ((not (pair? tree)) (list tree))
        (else (append (enumerate-tree (car tree))
                    (enumerate-tree (cdr tree))))))

(define (count-leaves t)
    (accumulate + 0 (map (lambda (x) 1) (enumerate-tree t))))

(define x (cons (list 1 2) (list 3 4)))
(count-leaves x)