Exercise 2.3.  Implement a representation for rectangles in a plane. (Hint: You 
may want to make use of exercise 2.2.) In terms of your constructors and 
selectors, create procedures that compute the perimeter and the area of a given 
rectangle. Now implement a different representation for rectangles. Can you 
design your system with suitable abstraction barriers, so that the same 
perimeter and area procedures will work using either representation?

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; storing a rectangle as two opposite corner points

(define (make-rect p1 p2) (cons p1 p2))

(define (p1-rect rect) (car rect))
(define (p2-rect rect) (cdr rect))

; height and width

(define (abs-diff x y) (abs (- x y)))

(define (height rect) (abs-diff (y-point (p1-rect rect)) (y-point (p2-rect rect))))

(define (width rect) (abs-diff (x-point (p1-rect rect)) (x-point (p2-rect rect))))

; perimeter and area

(define (perimeter rect)
  (* 2 (+ (height rect) (width rect))))

(define (area rect)
  (* (height rect) (width rect)))

; an alternative representation as a pair of xs and a pair of ys
; we use the same interface so all the other code above will still work

(define (make-rect p1 p2)
  (cons (cons (x-point p1) (x-point p2))
        (cons (y-point p1) (y-point p2))))

(define (p1-rect rect)
  (make-point (car (car rect)) (car (cdr rect))))

(define (p2-rect rect)
  (make-point (cdr (car rect)) (cdr (cdr rect))))