anchor.diff:      (let ((res +nowhere+))
         (loop for (r) in erase-overlapping do (setf res (region-union res r)))
         (loop for (r) in move-overlapping do (setf res (region-union res r)))
  -      (replay history stream res)) ))
  +      (replay history stream res))))
   
   (defclass updating-stream-state (complete-medium-state)
     ((cursor-x :accessor cursor-x :initarg :cursor-x :initform 0)
  
positive.code:                        (dy (- cy sy)))
                     (unless (zerop dy)
                       (move-output-record record dx dy) )
                     (let ((tag (cond ((= dx dy 0) :clean)
                                      (t :moved))))
                       (setf (output-record-dirty record) tag)
                       (setf (output-record-parent record) nil)
                       (map-over-updating-output #'(lambda (r)
  
negative.code:     ((record standard-updating-output-record) stream displayer)
    (multiple-value-bind (x y)
        (output-record-position record)
      (let ((sub-record (sub-record record)))
        (when sub-record
          (delete-output-record sub-record record)))
      ;; Don't add this record repeatedly to a parent updating-output-record.
      (unless (eq (output-record-parent record)
  		(stream-current-output-record stream))
  