public function includeXSL($files, $prepend = false)
    {
        $files = (array) $files;

        foreach ($files as $file) {
            if (!in_array($file, $this->xslfiles, true)) {
                if ($prepend) {
                    array_unshift($this->xslfiles, $file);
                } else {
                    $this->xslfiles[] = $file;
                }
            }
        }
    }