anchor.diff:  (defmethod alarm ((thopter thopter) event)
     (shoot thopter event)
     (setf (timer thopter) 4))
  +
  +(defmethod toggle-boost ((thopter thopter) event)
  +  (if (boost-on thopter)
  +      (progn (setf (boost-on thopter) nil) (change-veloc thopter event))
  +      (setf (boost-on thopter) t)))
   
   (defmethod initialize-instance :after ((shooter shooter) &key)
     (setf (ammo shooter) (* (ammo shooter) (ammo-deplete-rate shooter))))
  
positive.code:   (bind-key-down thopter :sdl-key-lalt  #'missile)
    (bind-key-down thopter :sdl-key-rctrl #'missile)
    (bind-key-down thopter :sdl-key-ralt  #'missile)
    ;; TODO: add reset function
    ;(bind-key-down thopter :sdl-key-r     #'reset)
    )
  
negative.code: 
  (defmethod collide ((thopter thopter) event)
    (with-slots (parent offset depth veloc health ammo ammo-refill-rate 
                        missiles speed-boost) thopter
      (typecase (event-hit event)
        (enemy-bullet  (decf health))
        (enemy-missile (decf health))
  