;Подсчёт листьев дерева

(define (leafs tree)
  (if (null? tree)
      '()
      (if (pair? tree)
          (append (leafs (car tree)) (leafs (cdr tree)))
          (list tree))))

(define (count-leaves tree)
  (foldr (lambda (x accum) (+ (length x) accum))
         0
         (map leafs tree)))