protected function _normalize($key, $type)
    {
        $type = strtolower($type);

        if ($type !== 'external' && $type !== 'internal')
            throw new \InvalidArgumentException;

        if ($type === 'external')
            $return = $this->_normalizeExternal($key);
        else
            $return = $this->_normalizeInternal($key);

        return $return;
    }