Some the typical attribute values the hierarchy

136	FRAME-BASED EXPERT SYSTEMS	-	-
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Figure 5.3 Inheritance of slot values in a simple frame structure: (a) relations of the car frames; (b) car frames and their slots

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Relationships between frames in such a hierarchy constitute a process of specialisation. The class-frame on the top of the hierarchy represents some generic concept, class-frames further down stand for a more restricted concept and the instances are closer to exemplification.

How are objects related in a frame-based system? Is the ‘is-a’relationship the only one available to us?

Unlike generalisation and aggregation relationships, associations usually appear as verbs and are inherently bi-directional.

Does a computer own Mr Black? Of course, the name of a bi-directional association reads in a particular direction (Mr Black owns a computer), but this direction can be changed to the opposite. The inverse of owns is belongs to, and thus we can anticipate that a computer belongs to Mr Black. In fact, both directions are equally meaningful and refer to the same association.