anchor.diff:  	(first-time t))
       (map-over-output-records
        (lambda (child)
  -       (if first-time
  -           (progn
  -             (multiple-value-setq (new-x1 new-y1 new-x2 new-y2)
  -               (bounding-rectangle* child))
  -             (setq first-time nil))
  -           (with-bounding-rectangle* (cx1 cy1 cx2 cy2) child
  -             (minf new-x1 cx1)
  -             (minf new-y1 cy1)
  -             (maxf new-x2 cx2)
  -             (maxf new-y2 cy2))))
  +       (unless (null-bounding-rectangle-p child)
  +         (if first-time
  +             (progn
  +               (multiple-value-setq (new-x1 new-y1 new-x2 new-y2)
  +                 (bounding-rectangle* child))
  +               (setq first-time nil))
  +             (with-bounding-rectangle* (cx1 cy1 cx2 cy2) child
  +               (minf new-x1 cx1)
  +               (minf new-y1 cy1)
  +               (maxf new-x2 cx2)
  +               (maxf new-y2 cy2)))))
        record)
       (if first-time
   	;; XXX banish x y
  
positive.code:             ;; In the following cases, we can grow the new bounding rectangle
              ;; from its previous state:
              ((or
                ;; If the child was originally empty, it should not have affected
                ;; previous computation of our bounding rectangle.
                ;; This is hackish for reasons similar to the above.
                (and (zerop old-min-x) (zerop old-min-y)
                     (zerop old-max-x) (zerop old-max-y))
  	      ;; For each old child coordinate, either it was not
  	      ;; involved in determining the bounding rectangle of the
  	      ;; parent, or else it is the same as the corresponding
  	      ;; new child coordinate.
  	      (and (or (> old-min-x ox1) (= old-min-x cx1))
  		   (or (> old-min-y oy1) (= old-min-y cy1))
  		   (or (< old-max-x ox2) (= old-max-x cx2))
  
negative.code: 						  ox1 oy1 ox2 oy2)))))))
    record)
  
  (defmethod tree-recompute-extent ((record compound-output-record))
    (tree-recompute-extent-aux record)
    record)
  