anchor.diff:        (upgrade-bullet (incf firepower)))
       (when (and parent (<= health 0))
         (make-instance 'explosion :parent parent
  -                     :offset offset :depth depth :veloc veloc
  +                     :offset offset :depth depth :veloc (/ veloc 2)
                        :image (make-instance 'image :name :explosion)
                        :timer 10)
         (detach parent thopter))))
  
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:   (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)) 90)))
  
  (defmethod alarm ((wave wave-controller) event)
    (with-slots (level) wave
  