XML Topic Maps: Creating and Using Topic Maps for the Web

Class Hierarchies

In our approach to topic map schemas, all topics, occurrences, and associations are instances of classes. The classes themselves are expressed as topics. The class–instance relationship declared by the <instanceOf> element is in fact merely a syntactically privileged association class defined in the text of the XTM 1.0 specification. If we are looking at the class–instance relation from an ontology/taxonomy view, then there is a justifiable demand for a superclass–subclass relationship as well.

Our example contains some superclass–subclass relationships:

• Company → customer, owner

• Employee → team member

• Team member → project manager

• Technology → product, standard

The XTM specification predefines PSIs that specify both superclass–subclass ...