anchor.diff:             :documentation "The key is an owner identifier (an artitrary balue),
   the value is an `owner'.")
      (current-owner (error "Owner required")
  +                  :reader current-owner
                     :type owner
                     :documentation "Must be one of the `owners' values.")
      (entries (make-entry-hash-table)
  
positive.code:                  (apply (if flatten #'append #'(lambda (&rest args) args))
                          (mapcar #'traverse (funcall children-function node)))))))
      (let ((root (typecase tree
                    (htree:node tree)
                    (htree:history-tree (htree:root tree)))))
        (when root
          (if include-root
              (traverse root)
  
negative.code:     (unless owner
        (setf owner
              (setf (gethash owner-identifier (owners history)) (make-instance 'owner))))
      (setf (slot-value history 'current-owner) owner)))
  
  (export-always 'current-owner-node)
  ;; (declaim (ftype (function (history-tree) (or null node)) current-owner-node)) ; TODO: Fix type.
  