private static function retrieve_defaults()
 {
     if (!self::$from_currency_code) {
         if (self::$from_currency_code = Session::get("CurrencyConverter.from_currency_code")) {
         } else {
             self::$from_currency_code = self::$defaults["DefaultFromCurrency"];
         }
         if (!self::$from_currency_code && self::$to_currency_code) {
             self::$from_currency_code = self::$to_currency_code;
         }
     }
     if (!self::$to_currency_code) {
         if (self::$to_currency_code = Session::get("CurrencyConverter.to_currency_code")) {
         } else {
             self::$to_currency_code = self::$defaults["DefaultToCurrency"];
         }
         if (self::$from_currency_code && !self::$to_currency_code) {
             self::$to_currency_code = self::$from_currency_code;
         }
     }
 }