The duality of relations below corresponds to the duality `inheritance' and `delegation' we
have just discussed.
This relation has to do with inheritance: A dog is an animal. A car is a vehicle.
Because a dog is animal it has all the features of animal and of course on top of that its
In the system using delegation described above we have seen that an object must have an
instance of its super-class in its attributes in order to be able to delegate an unknown
message to a higher level.
Of course an object may have instances of several super-classes making it easy to implement