anchor.diff:    (:export-class-name-p t)
     (:export-accessor-names-p t)
     (:accessor-name-transformer #'class*:name-identity)
  -  (:documentation "The relation ship between an owner and the current node."))
  +  (:documentation "The relationship between an owner and the current node."))
   
   (define-class owner-header ()
     ((origin nil ; TODO: Rename to `root'?  Not to be confused with the htree root, but maybe it's convenient to have the same method name.
  
positive.code:             :type (or null node)
              :initform nil
              :documentation "The current node.
  It changes every time a node is added or deleted.")
     (nodes (make-hash-table)
            :type hash-table
            :documentation "The set of all unique nodes visited by an owner."))
  
negative.code: 
  (declaim (ftype (function (history-tree t) owner-header) set-current-owner))
  (defun set-current-owner (history owner)
    "OWNER is arbitrary data representing an owner, it's not a `owner-header' object."
    (let ((header (gethash owner (owners history))))
      (unless header
        (setf (gethash owner (owners history)) (make-instance 'owner-header)))
  