anchor.diff:    (when (and (zerop (iter (for i in-vector (children (game-root game)))
                             (when (typep i 'enemy) (count i))))
                (not (timer (game-wave game))))
  -    (setf (timer (game-wave game)) 30)))
  +    (setf (timer (game-wave game)) 90)))
   
   (defmethod alarm ((wave wave-controller) event)
     (with-slots (level) wave
  
positive.code:     (when drop-class
        (make-instance drop-class :parent parent
                       :offset (+ offset (/ (- size (size drop-image)) 2))
                       :depth depth :veloc (/ veloc 2) :image drop-image))
      (detach parent explosion)))
  
  (defmethod collide ((health health-pack) event)
  
negative.code:       (upgrade-bullet (incf firepower)))
      (when (and parent (<= health 0))
        (make-instance 'explosion :parent parent
                       :offset offset :depth depth :veloc (/ veloc 2)
                       :image (make-instance 'image :name :explosion)
                       :timer 10)
        (detach parent thopter))))
  