The different approaches to generalization will be discussed in the context of an example system called the Object Oriented Content System (OOC). It is a research system being developed at the University of North Carolina by one of the authors (Smith) to explore the application of O-O concepts to documents and other forms of information. Its data model includes a concept of Set that can be applied to a variety of specific data types. Sets are also recursive so that Sets may include other Sets a members. Thus, one may use Sets to define a variety of different hierarchies, e.g., content objects, attributes, people, etc.
In the examples discussed here, the superclass for a given layer will include OOC as a part of its name, indicating that it is general for that layer. Design considerations for subclassing the superclass for a layer will be discussed with regard to the specific subclass for Sets within that layer.