public static function addPluginStyleSheet($folder, $plugin, $stylesheet = '', $dir = 'css')
	{
		if ($dir != 'css')
		{
			$stylesheet = $dir . '/' . $stylesheet;
		}

		$asset = new Stylesheet('plg_' . $folder . '_' . $plugin, $stylesheet);

		if ($asset->exists())
		{
			if ($document = self::app('document'))
			{
				$document->addStyleSheet($asset->link());
			}
		}
	}