void HandleCurrentGroupIsClosed()
{
Debug.Assert( _currentIsGroupClosed && _path.Count > 0 );
_current = null;
_path.RemoveAt( _path.Count - 1 );
if( _path.Count > 0 ) _current = _path[_path.Count - 1];
_currentIsGroup = _current != null;
_currentIsGroupClosed = false;
}