anchor.diff:              :type (or null node)
               :initform nil
               :documentation "The current node.
  -It changes every time a node is added or deleted.")
  +It's updated every time a node is visited.")
      (nodes (make-hash-table)
             :type hash-table
             :documentation "The set of all unique nodes visited by an owner."))
  
positive.code: 
  (declaim (ftype (function (history-tree t) owner-header) set-current-owner))
  (defun set-current-owner (history owner)
    (let ((header (gethash owner (owners history))))
      (unless header
        (setf (gethash owner (owners history)) (make-instance 'owner-header)))
  
negative.code:   (:export-class-name-p t)
    (:export-accessor-names-p t)
    (:accessor-name-transformer #'class*:name-identity)
    (: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.
  