anchor.diff:  		 
   
   (defparameter *G* 6.67e-11) ; Gravitational Constant
  -(defparameter *screen-size* (make-instance 'point :x 320 :y 320))
  +(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 
  
positive.code:               (y *screen-size*)
                :title-caption "OrbSim Prototype v1.09e-23")
      (setf (sdl:frame-rate) 60))
  
  (defun sdl-main-loop ()
    ; Define key events
  
negative.code: 			    'nil))
  	  *bodies*))
  
  (defmacro edit-parameter (id parameter value)
    `(setf (,parameter (nth (position ,id
  				    (mapcar #'(lambda (x) (id x)) *bodies*))
  			  *bodies*))
  	 ,value))
  
  (defun sdl-init ()
    "Initialize SDL environment"
  