public function generate($doClean = true) {
        // first, check if a custom template is set
        $customTemplate = $this->_getCustomTemplateFilename();
        if ($customTemplate) { 
            $tplFile = $customTemplate;
        } else { 
            $tplFile = $this->_getVclTemplateFilename(self::VCL_TEMPLATE_FILE);
        }
        $vcl = $this->_formatTemplate(file_get_contents($tplFile),
            $this->_getTemplateVars());
        return $doClean ? $this->_cleanVcl($vcl) : $vcl;
    }