;Segments usage procedures
(define (midpoint-segment s)
  (let ((start-p (start-segment s))
        (end-p (end-segment s)))
       (make-point
         (/ (+ (x-point start-p) (x-point end-p)) 2)
         (/ (+ (y-point start-p) (y-point end-p)) 2))))

; Segments
(define (make-segment start-p end-p) (cons start-p end-p))
(define (start-segment s) (car s))
(define (end-segment s) (cdr s))

; Points
(define (make-point x y) (cons x y))
(define (x-point p) (car p))
(define (y-point p) (cdr p))