anchor.diff:            :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'."))
  +  (: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)
  
positive.code:   (:export-class-name-p t)
    (:export-accessor-names-p t)
    (:accessor-name-transformer #'class*:name-identity)
    (:documentation "Node structure of the history tree."))
  
  (export 'data)
  (defmethod data ((node node))
  
negative.code:   (:export-class-name-p t)
    (:export-accessor-names-p t)
    (:accessor-name-transformer #'class*:name-identity)
    (: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."
  