code: public static BinaryOperator<MonetaryAmount> sum(
  			ExchangeRateProvider provider, CurrencyUnit currency) {
  		CurrencyConversion currencyConversion = provider
  				.getCurrencyConversion(currency);
  
  		return (m1, m2) -> currencyConversion.apply(m1).add(
  				currencyConversion.apply(m2));
  	}
docs: return the sum and convert all values to specific currency using the
  provider, if necessary
  @param provider the rate provider to be used, not null.
  @param currency
  currency
  @return the list convert to specific currency unit