anchor.diff:                                              ((upgrade-bullet) :upgrade-bullet)
                                               ((health-pack) :health)))))
           (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
                          :drop-class drop-class :drop-image drop-image)
  
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))))
  