anchor.diff:                       :parent parent 
                        :offset (+ offset (/ (x size) 2) #c(0 -4)) :depth -1
                        :veloc (+ veloc #c(0 -8))
  -                     :image (make-instance 'image :name :missile)))))
  +                     :image (make-instance 'image :name :missile)
  +                     :timer 120))))
   
   (defmethod update ((missile missile) event)
     (with-slots (parent offset veloc accel) missile
  
positive.code:   (with-slots (parent offset size depth veloc) missile
      (when (and parent (or (typep (event-hit event) 'enemy)
                            (typep (event-hit event) 'explosion)))
        (let ((explosion (make-instance 'image :name :explosion)))
          (make-instance 'explosion :parent parent
                         :offset (+ offset (/ size 2) (/ (size explosion) -2))
  
negative.code:     :initarg :missiles
      :initform 0)))
  (defclass bullet (sprite mobile collidable transient) ())
  (defclass missile (sprite mobile collidable alarm) ())
  (defclass enemy (sprite mobile collidable alarm shooter)
    ((timer :initform 10)
     (bullet-class :initform 'enemy-bullet)
  