public function exists($name)
    {
        list($key, $extra) = $this->_splitOptionName($name);
        if (isset($this->_values[$key]) && is_array($this->_values[$key]) && array_key_exists($extra, $this->_values[$key])) {
            return true;
        }
        if (isset($this->_options[$name])) {
            return isset($this->_options[$name]['default']);
        }
        return false;
    }