Aggregation is the creation of a unique (aggregate) of the object corresponding to each non-NULL value of some aggregated property. Each such object is assumed to have properties that map this object to each aggregated property parameter, and, conversely, a property that maps the aggregated property parameters to this object.
The aggregated object and each aggregated property parameter must belong to a specified class.
The aggregation mechanism is implemented using two consequences with automatic resolution and an aggregation operator. With the help of the aggregation operator, the first consequence creates an object when the aggregated property becomes non-NULL, and writes the necessary values to all its properties. The second consequence deletes the object when the aggregated property becomes NULL.
To create aggregations, use the operatorAGGR.