SICP 2.2.part2

(define (make-segment p1 p2) (cons p1 p2))
(define (start-segment s) (car s))
(define (end-segment s) (cdr s))
(define (mid-segment s)
  (let ((x1 (x-point (start-segment s)))
        (y1 (y-point (start-segment s)))
        (x2 (x-point (end-segment s)))
        (y2 (y-point (end-segment s))))   
  (make-point (/ (+ x1 x2) 2) (/ (+ y1 y2) 2))))
(define (print-segment s)
  (print-point (start-segment s))
  (display ",")
  (print-point (end-segment s)))