anchor.diff:  (defmethod activate :after ((screen thopter-play-screen))
     (iter (for i in-vector (children (game-root screen)))
           (when (typep i 'thopter)
  -          (clear-flags i)
  -          (change-veloc i nil)))
  +          (stop-all i nil)))
     ;; Now is a relatively good time to do garbage collection.
     (gc :full t))
   
  
positive.code: 
  (defun quadrant (x n)
    (mod (floor (+ (/ (* (theta x) n) (* 2 pi)) 0.5d0)) n))
  
  (defmethod start-shoot ((weapon weapon) event)
    (shoot weapon event)
  
negative.code:         (bind-key-up thopter k (doall (set-flag d nil) #'change-veloc)))
    (bind-key-down thopter :sdl-key-space #'start-shoot-primary)
    (bind-key-up thopter :sdl-key-space #'stop-shoot-primary)
    (iter (for k in '(:sdl-key-lctrl :sdl-key-lalt
                      :sdl-key-rctrl :sdl-key-ralt))
          (bind-key-down thopter k #'start-shoot-secondary)
          (bind-key-up thopter k #'stop-shoot-secondary))
    (bind-key-down thopter :sdl-key-lshift #'toggle-boost)
    (bind-key-down thopter :sdl-key-rshift #'toggle-boost)
    ;; TODO: add reset function
  