anchor.diff:    (:accessor-name-transformer #'class*:name-identity)
     (:documentation "Starting point of the global history tree data structure."))
   
  +(export-always '+default-owner+)
   (defparameter +default-owner+ "default-owner")
   
   (export 'make)
  
positive.code: 
  
  (export-always 'add-child)
  (defmethod add-child (data (history history-tree) &key creator)
    "Create or find a node holding DATA and set current node to it.
  Return the (possibly new) current node.
  
  
negative.code:   ((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.
             :type (or null node)
             :documentation "The first node created for this owner.")
     (creator nil                         ; TODO: Rename to `creator-identifier'?
              :type t
              :documentation "The owner-identifier in `origin's parent node that
  created this owner.  May be nil, in which case `origin' is a root node.
  