1 (defun make-segment (start end)
2   (cons start end))
3 (defun start-segment (segment)
4   (car segment))
5 (defun end-segment (segment)
6   (cdr segment))
7 (defun make-point (x y)
8   (cons x y))
9 (defun x-point (point)
10   (car point))
11 (defun y-point (point)
12   (cdr point))
13 (defun print-point (point)
14   (format t "(~F,~F)~%" (x-point point) (y-point point)))
15 (defun midpoint-segment (segment)
16   (let ((segstart (start-segment segment))
17 	(segend (end-segment segment)))
18     (make-point (average (x-point segstart)
19 			 (x-point segend))
20 		(average (y-point segstart)
21 			 (y-point segend)))))
22 (defvar aa (make-point 4 6))
23 (defvar bb (make-point 9 15))
24 (print-point
25  (midpoint-segment (make-segment aa bb)))
26