练习2.51
通过书中前面的内容，我们知道below中来将框架分成上下两部分，而beside将框架分成左右两部分。因此，below定义如下：
(define (below painter1 painter2)
(let ((split-point (make-vect 0.0 0.5)))
(let ((paint-top (transform-painter painter2
split-point
(make-vect 1.0 0.5)
(make-vect 0.0 1.0)))
(paint-down (transform-painterpainter1
(make-vect 0.0 0.0)
(make-vect 1.0 0.0)
split-point)))
(lambda (frame)
(paint-top frame)
(paint-down frame)))))
另一种利用上一题中的rotate270等可以很容易的实现：
(define (below painter1 painter2)
(lambda (frame)
((flip-horiz
(rotate90
(beside
(rotate270
(flip-horizpainter1))
(rotate270
(flip-horizpainter2)))))
frame)))
原文地址：http://blog.csdn.net/nomasp/article/details/43907459
