(defun midpoint (pt1 pt2 cx cy cz /)
(list 
	(+ (/ (+ (car pt1) (car pt2)) 2) cx) 
	(+ (/ (+ (cadr pt1) (cadr pt2)) 2) cy) 
	(+ (/ (+ (caddr pt1) (caddr pt2)) 2)) cz)
)