The duality of relations below corresponds to the duality `inheritance' and `delegation' we
have just discussed.
IS-Relation
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
own.
HAS-Relation
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
multiple inheritance.
