public static MoneyValue Sum <T>(this IEnumerable <T> source, Func <T, MoneyValue> selector)
 {
     return(MoneyValue.Of(source.Select(selector).Aggregate((x, y) => x + y)));
 }
 public static MoneyValue Of(MoneyValue value)
 {
     return(new MoneyValue(value.Value, value.Currency));
 }