anchor.diff:    (update-vel body)
     (update-pos body))
   
  -(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 draw-body (body)
  +  "Draws a body to the screen"
     (sdl-gfx:draw-filled-circle
      (pos2pos (pos body)) (size body) :color (colour body)))
   
  
positive.code:     (sets #'+ (y (vel body)) (y accel))))
  
  (defun update-pos (body)
    (sets #'+ (x (pos body)) (x (vel body)))
    (sets #'+ (y (pos body)) (y (vel body))))
  
  
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
  