anchor.diff:      (unless owner
         (setf owner
               (setf (gethash owner-identifier (owners history)) (make-instance 'owner))))
  -    (setf (current-owner history) 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.
  
positive.code:            :documentation "The key is an owner identifier (an artitrary balue),
  the value is an `owner'.")
     (current-owner (error "Owner required")
                    :type owner
                    :documentation "Must be one of the `owners' values.")
     (entries (make-entry-hash-table)
  
negative.code:   (let ((owner (owner owner-identifier history)))
      (remhash owner-identifier (owners history))
      (when (eq owner (current-owner history))
        (setf (slot-value history 'current-owner)
              (first-hash-table-value (owners history))))
      (when owner
        (mapc
  