anchor.diff:  (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 55 
  -				   :mass 5.97e24 :size 3 :colour sdl:*blue*))
  +				 :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 *bodies* (list *sun* *earth*))
  +(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"
  
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:       (:idle ()  
         (sdl:clear-display sdl:*black*)
         
         (update-lst (cdr *bodies*))
          
         (draw-bodies *bodies*)
  
  