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When talking about inheritance, we only concentrate on contentful issues relating to the expres- sive power of the ontology metalanguage. We see ontological semantics as guided by the theory

of inheritance (e.g., Touretzky 1984, 1986, Thomason et al. 1987, Touretzky et al. 1987, Thoma- son and Touretzky 1991) but do not aspire to contributing to further development of the theory of inheritance. Our approach to inheritance is fully implementation-oriented.

The inheritance hierarchy, which is implemented using IS-A and SUBCLASSES slots, is the back- bone of the ontology. When two concepts, X and Y, are linked via an IS-A relation (that is, X IS-A Y), then X inherits slots (with their facets and fillers) from Y according to the following rules: • All slots that have not been overtly specified in X, with their facets and fillers, but are

specified in Y, are inherited into X.

• ONTOLOGY-SLOTs (IS-A, SUBCLASSES, DEFINITION, TIME-STAMP, INSTANCE-OF, INSTANCES, INVERSE, DOMAIN, RANGE) are excluded from this rule. They are not inherited from the parent.

• If a slot appears both in X and Y, then the filler from X takes precedence over the fillers from Y.

• Use the filler NOTHING to locally block inheritance on a property. If a parent concept has a slot with some facets and fillers and if some of its children have NOTHING as the filler of the SEM facet for that same slot, then the slot will not be inherited from the parent. Since the local slot in the child has NOTHING as its filler, no instance of any OBJECT or EVENT or any number or literal will match this symbol. As such, no filler is acceptable to this slot and this slot will never be present in any instance of this concept. This has the same effect as

removing the slot from the concept. For example, ANIMAL has the property MATERIAL-OF filled by AGRICULTURAL-PRODUCT; HUMAN IS-A ANIMAL and it inherits the slot MATERIAL-OF from ANIMAL; however, the filler of this slot in HUMAN is, for obvious reasons, NOTHING. Note that in descendants of HUMAN it is entirely possible to reintroduce fillers other than NOTHING in the MATERIAL-OF slot, for instance, in news reports about transplants or cloning. • Block the inheritance of a filler that is introduced through the NOT facet. Thus, the filler

HUMAN will be introduced through the facet NOT in the THEME slot of BUY, while the SEM facet will list OBJECT as its filler (and HUMAN is a descendant of OBJECT). This is our way of saying that, in the extant implementations of ontological semantics, people cannot be bought or sold (which, incidentally, may turn out to be a problem for processing news reports about slavery in the Sudan or buying babies for adoption).

Regular inheritance of a slot simply incorporates all fillers for the slot from all ancestors (con- cepts reached over the IS-A relation) into the inheriting concept. For example, a kitchen has a stove, a refrigerator, and other appliances. A room has walls, a ceiling, a floor, and so on. A kitchen, being a room, has the appliances as well as a floor, etc. Blocking inheritance indicates that a slot or slot/filler combination that appears in an ancestor should not be incorporated into the inheriting concept.

There are two reasons for blocking inheritance using NOTHING. First, in a subtree in the ontology, all but a few concepts might have a particular property (slot). It is much easier to put the slot at the root of the subtree and block it in those few concepts (or subtrees) which do not have that slot rather than putting the slot explicitly in each of the concepts that do take the slot. For example, all EVENTS take the agent slot except PASSIVE-COGNITIVE-EVENTs and INVOLUNTARY-PERCEPTUAL-

EVENTs. We can put the AGENT slot (with the SEM constraint ANIMAL) in EVENT and put a SEM NOTHING in PASSIVE-COGNITIVE-EVENT and INVOLUNTARY-PERCEPTUAL-EVENT. This will effec- tively block the AGENT slot in the subtrees rooted under these two classes of EVENT while all other EVENTs will still automatically have the AGENT slot.

A second, stronger reason for introducing this mechanism comes from the needs of lexical seman- tics. Sometimes, the SEM-STRUC zone of the lexicon entry for certain words (see Section 7.2) will have to refer to a property (slot) defined for an entire class of concepts, even though a few con- cepts in that class do not actually feature that property. For example, in the SEM-STRUC of the Spanish activo, we must refer to the AGENT of EVENT without knowing what EVENT it is. This requires us to add an AGENT slot to EVENT even though there are two subclasses of EVENT that do not have AGENT slots. An alternative would be to list every type of EVENT other than the above two in the SEM-STRUC of the lexicon entry for this word. This, however, is not practical at all. In a sense, this mechanism is introducing the power of default slots just like we have a DEFAULT facet in a slot. We can specify a slot for a class of concepts which acts like a default slot: it is present in every concept unless there is an explicit SEM NOTHING filler in it.

While multiple inheritance is allowed and is indicated by the presence of more than one filler in the IS-A slot in a concept, no extant implementation of ontological semantics has fully developed sufficiently formal methods for using multiple inheritance.