Exercise 2.35.  Redefine count-leaves from section 2.2.2 as an 
accumulation:

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

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(define (count-leaves t)
  (accumulate + 0 (map (lambda (x) (if (pair? x) (count-leaves x) 1))
                       t)))