In certain situations, you may want to use the same name for different system properties or actions. 

For example, let's create two properties named sum: one of them will calculate the sum of the order line, and the second will calculate the sum of the entire order:

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 sum = DATA NUMERIC[10,2] (OrderDetail);
sum = GROUP SUM sum(OrderDetail od) BY order(od);

Accordingly, the first property gets one parameter of the OrderDetail class as input while the second gets one parameter of the Order class.

But if we create, for example, a constraint with one parameter and then try to refer to the sum property without explicitly specifying the class of this parameter, an error will occur:

All such references require an explicitly specified class:

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 CONSTRAINT sum(Order o) < 0 MESSAGE 'The order amount must be positive';