anchor.diff:  It only changes when deleted.")
      (owners (make-hash-table)
              :type hash-table
  -           :documentation "The key is an owner, the value is an `owner-header'."))
  +           :documentation "The key is an owner, the value is an `owner-header'.")
  +   (current-owner-header nil
  +                         :type t
  +                         :documentation "Must be one of the `owners' values."))
     (:export-class-name-p t)
     (:export-accessor-names-p t)
     (:accessor-name-transformer #'class*:name-identity)
  
positive.code: (defun make ()
    (make-instance 'history-tree))
  
  (defmethod (setf current) (value (history history-tree))
    (if value
        (setf (slot-value history 'current) value)
        (error "Attempted to set current history-tree node to NIL.")))
  
  
  
  (deftype positive-integer ()            ; TODO: Remove if unused.
    `(integer 1 ,most-positive-fixnum))
  
negative.code:   (:accessor-name-transformer #'class*:name-identity)
    (:documentation "The high-level information about an owner."))
  
  (defmethod (setf current) (value (header owner-header))
    (if value
        (setf (slot-value header 'current) value)
        (error "Attempted to set current node to NIL for owner header ~a." header)))
  
  (define-class history-tree ()
    ((root nil
           :type (or null node)
  