anchor.diff:  ;; TODO: Is "Shared history tree" a better name than "Global history tree"?
   
   ;; TODO: Turn unique defmethod to defuns.
  +;; TODO: Thread safe?
  +
   ;; TODO: Add forward and back functions to unowned nodes.
  -;; TODO: Thread safe?
  -
   ;; TODO: Should we have different functions for finding nodes vs. "owned nodes",
   ;; or pass an option as key argument?
   
  
positive.code: (defun owned-p (owner node)
    (and (bindings node)
         (gethash owner (bindings node))))
  
  (declaim (ftype (function (owner node) boolean) disown))
  (defun disown (owner node)
  
negative.code:               new-entry))))))
  
  (define-class history-tree ()           ; TODO: Rename `history'?
    ((owners (error "Owners need be initialized")
             :type hash-table
             :documentation "The key is an owner identifier (an artitrary balue),
  the value is an `owner'.")
  