有一个订单类
СLASS Order 'Заказ';
number 'Номер' = DATA STRING[10] (Order);
client = DATA Client (Order);
amount = DATA NUMERIC[17,2] (Order);
有一个“子表”类OrderProducts其实就是组成订单
CLASS OrderProducts 'Состав заказа';
product = DATA Product (OrderProducts);
productName 'Наименование' (OrderProducts d) = name(product(d));
qty = DATA NUMERIC[5,3] (OrderProducts);
price = DATA NUMERIC[17,2] (OrderProducts);
amount (OrderProducts a) = qty(a)*price(a);
如何计算订单金额并在表单上以金额（订单）显示？