Why complicate task models with specialization, composition, and extension?
Separating out the meaningful common and reusable parts of task cases ultimately simplifies the model overall. Specialization recognizes that some task cases are just specialized variants of other, more generic ones. Just as with inheritance in object-oriented design, we do not have to keep rewriting the generic parts every time we describe a specialized variation. Similarly, composition allows us to construct complicated task cases by having them use other task cases. Extension allows us to keep the base cases that represent the primary or normal course of events clean and simple by separating out the unusual or exceptional interactions.
