anchor.diff:        (:idle ()  
          (sdl:clear-display sdl:*black*)
          
  -       (update *earth*)
  +       (update-lst (cdr *bodies*))
           
          (draw-bodies *bodies*)
   
  
positive.code:   (update-vel body)
    (update-pos body))
  
  (defun draw-body (body)
    "Draws a body to the screen"
    (sdl-gfx:draw-filled-circle
     (pos2pos (pos body)) (size body) :color (colour body)))
  
  (defun draw-bodies (bodies)
    (mapc #'draw-body bodies))
  
  (defun draw-bodies (bodies)
    (if bodies
        (progn (draw-body (car bodies))
  	     (draw-bodies (cdr bodies)))
        't))
  
  (defun init ()
    "Initialize SDL environment"
  
negative.code: (defparameter *screen-size* (make-instance 'point :x 640 :y 640))
  
  (defparameter *earth* (make-body :pos-x 0 :pos-y 1e8 
  				 :vel-x 1e6 :vel-y 0 
  				 :mass 5.97e24 :size 3 :colour sdl:*blue*))
  (defparameter *sun* (make-body :pos-x 0 :pos-y 0 
  			       :mass 1.99e30 :size 10 :colour sdl:*yellow*))
  (defparameter *mars* (make-body :pos-x 0 :pos-y -7e7 
  				:vel-x -1e6 :vel-y 0
  				:size 2 :colour sdl:*red*))
  (defparameter *bodies* (list *sun* *earth* *mars*))
  
  (defun pos= (a b)
    "Checks if two points are equal"
  