protected static function addMap($mapfile)
    {
        if (! in_array($mapfile, self::$maps)) {
            // keep track of specific map file loaded in this
            // instance so we can update it if necessary
            self::$mapfile = $mapfile;

            if (file_exists($mapfile)) {
                $map = include $mapfile;
                if (is_array($map)) {
                    // mapfile contains a valid map, so we'll keep it
                    self::$maps[] = $mapfile;
                    self::$map = array_merge(self::$map, $map);
                }
            }
        }
    }