anchor.diff:      (sets #'+ (y (vel body)) (y accel))))
   
   (defun update-pos (body)
  +  "Updates the position of a body using its velocities"
     (sets #'+ (x (pos body)) (x (vel body)))
     (sets #'+ (y (pos body)) (y (vel body))))
   
  
positive.code:                  :mass mass
  		 :size size
  		 :colour colour))
  
  (defparameter *earth* (make-body :pos-x 0 :pos-y 1e8 
  				   :vel-x 1e6 :vel-y 55 
  				   :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 *screen-size* (make-instance 'point :x 640 :y 640))
  (defparameter *G* 6.67e-11)
  (defparameter *bodies* '(*sun* *earth*))
  
  (defun pos= (a b)
  
negative.code: (defun draw-bodies (bodies)
    (loop for bod in bodies
         do (draw-body bod)))
         
  (defun init ()
    "Initialize SDL environment"
    (sdl:window (x *screen-size*)
                (y *screen-size*)
                :title-caption "OrbSim Prototype v1.09e-23")
      (setf (sdl:frame-rate) 60))
  
  (defun main-loop ()
    ; Define key events
  