anchor.diff:  		 :size size
   		 :colour colour
   		 :id id))
  -		 
   
   (defparameter *G* 6.67e-11) ; Gravitational Constant
   (defparameter *screen-size* (make-instance 'point :x 640 :y 640))
  
positive.code: 		       :color sdl:*red*))
  	*a*))
  
  
  (defun sdl-main-loop ()
    ; Define key events
  
negative.code:        (update-lst (cdr *bodies*))
         (draw-bodies *bodies*)
         
  ;       (setf *a* (append *a* (list (pos2pos (pos (cadr *bodies*))))))
  ;       (draw-trails)
  	
        (sdl:update-display))))
  
  (defun main ()
    (sb-thread:make-thread 
     #'(lambda () (progn
  		  (sdl:with-init ()
  		    (sdl-init)
  		    (sdl-main-loop)))))
    (loop until (not *run*)
         do (progn
  	    (princ "OrbSim> ")
  	    (print (eval (read))))))
  