protected function addDir($dir)
    {
        // Add any additional directories to the top of the array
        // so that the user can override art
        array_unshift($this->art_dirs, rtrim($dir, '/'));

        // Keep the array clean
        $this->art_dirs = array_unique($this->art_dirs);
        $this->art_dirs = array_filter($this->art_dirs);
        $this->art_dirs = array_values($this->art_dirs);
    }