SICP 2.3.part2

(define (make-rect2 p1 p2)
  (let ((p3 (make-point (x-point p2) (y-point p1))))
  (cons (make-segment p1 p3) (make-segment p3 p2))))
(define (width2 r)
  (let ((seg (car r)))
    (- (x-point (end-segment seg)) (x-point (start-segment seg)))))
(define (length2 r)
  (let ((seg (cdr r)))
    (- (y-point (end-segment seg)) (y-point (start-segment seg)))))
(define (area2 r) (* (width2 r) (length2 r)))
(define (perimeter2 r) (* (+ (width2 r) (length2 r)) 2))