The Relation Pattern

When a slot in an EVENT or OBJECT is a Relation, the filler of the slot will be another EVENT or OBJECT. This filler should have a corresponding link to the first concept through a different slot. This slot will also be a Relation that is an Inverse of the first Relation. This pattern is shown in a simplified form in Figure 1 . The first two slots are simply labeled Property. Only the third slot which is a Relation is expanded. It may be noted that the Relation itself is defined as a concept in the ontology. This Relation, shown in the bottom half of the figure, has only Domain, Range, and Inverse slots.

   
Figure 1: The Relation Pattern.

Additional complexities arise from the facts that the Inverse link may be

• inherited by the second concept from one of its ancestors rather than defined as a local slot in that concept, or
• to a concept that is an ancestor of the first concept, or
• implicit in the domain and range specifications of the properties.

The resulting complex patterns are illustrated in Figure 2 which shows patterns developing from both inheritance and implicit links. Part (a) of the Figure shows an inherited link that does not have a direct inverse. There is a pair of links, Head-of and Headed-by, between CORPORATION and PRESIDENT-CORPORATION. The two relations Head-of and Headed-by are inverses of each other. The concept BANK is a descendant of CORPORATION and inherits the Headed-by link to PRESIDENT-CORPORATION. Now, if we just look at BANK and PRESIDENT-CORPORATION, we find that BANK has a Headed-by link to PRESIDENT-CORPORATION but the inverse link is not to BANK but to its ancestor CORPORATION. Intuitively, we refer to a pattern such as the one in Part (a) of the Figure as an inheritance triangle.

   
Figure 2: Complex Relation Patterns

Inheritance triangles can occur on both sides of a pair of reciprocal links. For example, Part (b) of Figure 2 shows a pair of relations Corporate-assets and Corporate-asset-of linking the concepts CORPORATION and FINANCIAL-OBJECT. When these slots are inherited down to MONEY (via ASSET) and DRUGSTORE as shown, the resulting pattern has the shape of an X: MONEY has a link to CORPORATION but not vice versa; DRUGSTORE has a link to FINANCIAL-OBJECT but not vice versa. Nor is there any direct link between MONEY and DRUGSTORE. However, these links are implicitly present and a constraint that is looking for them will be satisfied through inheritance. For example, MONEY can fill the Corporate-assets slot of a DRUGSTORE.

A complex pattern resulting partly from inheritance and partly from implicit inverse links is shown in Part (c) of Figure 2 . Here, HUMAN has a slot Owner-of which is constrained to OBJECT. This link gets inherited, for example, to TERRORIST. However, there is no explicitly encoded inverse link from OBJECT to HUMAN. Instead, when we look at the property concept Owned-by (or its inverse Owner-of) and examine its Domain and Range slots, we find that they implicitly say that HUMAN can be Owned-by of an OBJECT. This information in property concepts is in fact extracted and used by the K analyzer to check constraints. Thus, an OBJECT can be Owned-by a TERRORIST although there is no such link from OBJECT to TERRORIST or to any of its ancestors.

Finally, Part (d) of Figure 2 shows another pattern similar to Part (c) but with redundant links. In this case, there is no direct link in either direction between EVENT and OBJECT as far as the pair Theme/Theme-of is concerned. These links are implicit in the Domain and Range slots of Theme and Theme-of. However, there is an explicit Theme-of slots in ANIMATE, a descendant of OBJECT. This link is redundant, but does not hurt either the consistency of the ontology or the analysis processes in any way. Similarly, there is a redundant Theme link from PHYSICAL-EVENT to OBJECT.

Because the tools we use for concept acquisition are limited in their ability to help us visualize these complex patterns, our ontology does contain several such redundant links, which are harmless for our purposes. It may also be noted that, when there is an explicit link, it takes precedence over the information in the corresponding Domain and Range slots. Typically, Domains and Ranges of properties contain the most general constraints; we add explicit links between concepts to encode a more specific constraint on a conceptual relation.