public static function isLive()
    {
        if (preg_match("/^(live_private_|test_private_)/", self::$apiKey)) {
            return true;
        } elseif (preg_match("/^sandbox_private_/", self::$apiKey)) {
            return false;
        } else {
            throw new Error\InvalidApiKey('Set correct secret key.');
        }
    }