Exercise 2.50.  Define the transformation flip-horiz, which flips 
painters horizontally, and transformations that rotate painters 
counterclockwise by 180 degrees and 270 degrees.

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(define (flip-horiz painter)
  (transform-painter painter
                     (make-vect 1.0 0.0)
                     (make-vect 0.0 0.0)
                     (make-vect 1.0 1.0)))

(define (rotate180 painter)
  (transform-painter painter
                     (make-vect 1.0 1.0)
                     (make-vect 0.0 1.0)
                     (make-vect 1.0 0.0)))

(define (rotate270 painter)
  (transform-painter painter
                     (make-vect 0.0 1.0)
                     (make-vect 0.0 0.0)
                     (make-vect 1.0 1.0)))