protected function _addon($addon, $data)
    {
        if (is_string($addon)) {
            $class = 'input-group-' . ($this->_isButton($addon) ? 'btn' : 'addon');
            $addon = $this->_templates->format('inputGroupAddon', [
                'class' => $class,
                'content' => $addon,
                'templateVars' => $data['templateVars'],
            ]);
        } else {
            $class = 'input-group-btn';
            $addon = $this->_templates->format('inputGroupAddon', [
                'class' => $class,
                'content' => implode('', $addon),
                'templateVars' => $data['templateVars'],
            ]);
        }

        return $addon;
    }