anchor.diff:    ((health
       :accessor health
       :initarg :health
  -    :initform 4)
  +    :initform 8)
      (timer :initform 120)))
   (defclass explosion (sprite mobile collidable alarm)
     ((drop-class
  
positive.code:     (incf level)
      (if (zerop (mod level 10))
          (spawn-wave level (truncate level 10) (* level 20) (* level 4)
  		    (truncate level 5) (min 1d0 (* level 0.02)) 'enemy-boss)
          (spawn-wave level (+ 2 level) 4 (max 1 (ceiling level 3)) 0
                      (min 1d0 (* level 0.01)) 'enemy-ship))))
  
  
negative.code:   (with-slots (parent offset size depth veloc health) missile
      (typecase (event-hit event)
        (bullet    (decf health))
        (missile   (setf health 0))
        (thopter   (setf health 0))
        (explosion (decf health)))
      (when (and parent (<= health 0))
        (let ((explosion (make-instance 'anim :name :explosion)))
  