anchor.diff:    (:export-class-name-p t)
     (:export-accessor-names-p t)
     (:accessor-name-transformer #'class*:name-identity)
  -  (:documentation "The high-level information about an owner."))
  +  (:documentation "The high-level information about an owner.
  +Each owner is identified by a unique identifier, which is arbitrary data (may
  +even be NIL)."))
   
   (defmethod (setf current) (value (owner owner))
     "This setter protects against setting OWNER's `current' slot to an invalid object."
  
positive.code:                 :documentation "Timestamp of the last access to this node by the
  owner."))
    (:accessor-name-transformer #'class*:name-identity)
    (:documentation "The relationship between an owner and one of its nodes."))
  
  (export-always 'last-access)
  (defmethod last-access ((binding binding))
  
negative.code:           :type list
            :documentation "The list of nodes that access an entry."))
    (:accessor-name-transformer #'class*:name-identity)
    (:documentation "Wrapped data as stored in `history-tree''s `entries'.
  Each entry has a unique datum.  Each `node' points to one entry.  Multiple nodes
  may point to the same entry.  Entries may also be node-less; they are kept
  around so that we can remember the data that was visited since the beginning of
  time.  Node-less entries are available for manual deletion with
  `delete-data'."))
  
  (defun ensure-timestamp (string-or-timestamp)
    (if (stringp string-or-timestamp)
  