anchor.diff:  
   (defmethod collide ((thopter thopter) event)
     (with-slots (parent offset depth veloc health ammo ammo-refill-rate 
  -                      missiles) thopter
  +                      missiles speed-boost) thopter
       (typecase (event-hit event)
         (enemy-bullet  (decf health))
         (enemy-missile (decf health))
  
positive.code: (defmethod alarm ((thopter thopter) event)
    (shoot thopter event)
    (setf (timer thopter) 4))
  
  (defmethod initialize-instance :after ((shooter shooter) &key)
    (setf (ammo shooter) (* (ammo shooter) (ammo-deplete-rate shooter))))
  
negative.code:             (incf (players-left (players-left screen)))))
      (let* ((tile-names '(:forest-0 :forest-1 :forest-2 :forest-3
                           :forest-4 :forest-5 :forest-6))
             (tiles (iter (for name in tile-names) (collect (make-image name))))
             (num-tiles (length tiles))
             (tile-size (size (first tiles))))
        (iter (for x from 0 below (x size) by (x tile-size))
              (iter (for y from 0 below (y size) by (y tile-size))
                    (make-instance 'sprite
                                   :parent root
                                   :offset (complex x y)
                                   :image (nth (mt19937:random num-tiles)
                                               tiles)
                                   :depth 100))))))
  