#lang sicp

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

(define (count-leaves t)
  (accumulate + 0 (map (lambda (sub-t)
                         (cond ((pair? sub-t) (count-leaves sub-t))
                               ((null? sub-t) 0)
                               (else 1)))
                       t)))