protected function setTilesClass($className) {
        if ($this->configured)                        throw new IllegalStateException('Configuration is frozen');
        if (!is_class($className))                    throw new StrutsConfigException('Class '.$className.' not found');
        if (!is_subclass_of($className, Tile::class)) throw new StrutsConfigException('Not a subclass of '.Tile::class.': '.$className);

        $this->tilesClass = $className;
    }