In this chapter we constructed a topic map that is capable of referencing other topic maps using the drill-down design pattern. We created a resource reference aimed at a particular topic map, and we used that reference in an occurrence of a topic contained within another topic map. For this scheme to work, the topic map engine that presents our topic map must be capable of identifying the occurrence type. To permit identification of the occurrence type, we reified our occurrence in a topic, which, itself, is an instance of another topic (in our example, the TopicMap topic).
 Clearly, there may be other coding styles with which a drill-down capability can be generated. The style presented here was selected for its simplicity.