17. Explain why the aggregation relation is a relation in the problem domain and not in the implementation domain.
There is no diﬀerence in the implementation of an aggregation relation and other association relations. In fact, it can be hard to decide if some relations are really aggregations or not. For example, it is obvious that a car is an aggregation of the car’s parts. However, it is not obvious whether a store should be considered an aggre- gation of customers.