Exercise 2.51.  Define the below operation for painters. Below takes two 
painters as arguments. The resulting painter, given a frame, draws with 
the first painter in the bottom of the frame and with the second painter 
in the top. Define below in two different ways -- first by writing a 
procedure that is analogous to the beside procedure given above, and 
again in terms of beside and suitable rotation operations (from exercise 
2.50).

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(define (below bottom top)
  (let ((split-point (make-vect 0.0 0.5)))
    (let ((paint-bottom
           (transform-painter bottom
                              (make-vect 0.0 0.0)
                              (make-vect 1.0 0.0)
                              split-point))
          (paint-top
           (transform-painter top
                              split-point
                              (make-vect 1.0 0.5)
                              (make-vect 0.0 1.0))))
      (lambda (frame)
        (paint-bottom frame)
        (paint-top frame)))))