public function setCurrency($country = null)
    {
        if (is_null($country)) {
            $country = config('currency.default');
        }
        $config = config('currency.' . $country);
        if (empty($config)) {
            throw new CurrencyNotAvaialbleException($country . ' currency not avaialble.');
        }
        $this->currency = $config;

        return $this;
    }