walker...to the next step

两个解法。注意lambda那个是如何等价于define的，尽管我们更习惯于define那个。

(define (split op1 op2)
  (define (op painter n)
    (if (= n 0)
      painter
      (let ((smaller (op painter (- n 1))))
        (op1 painter (op2 smaller smaller)))))
  op)

(define (split op1 op2)
  (lambda (painter n)
    (if (= n 0)
      painter
      (let ((smaller ((split op1 op2) painter (- n 1))))
        (op1 painter (op2 smaller smaller))))))