## Subsumption and Unification

It is standard to think of feature structures as providing partial information about some object, in the sense that we can order feature structures according to how general they are. For example, (25a) is more general (less specific) than (25b), which in turn is more general than (25c).

This ordering is called subsumption; a more general feature structure subsumes a less general one. If FSo subsumes FSi (formally, we write FSo E FSi), then FSi must have all the paths and path equivalences of FSo, and may have additional paths and equivalences as well. Thus, (23) subsumes (24) since the latter has additional path equivalences. It should be obvious that subsumption provides only a partial ordering on feature structures, since some feature structures are incommensurable. For example, (26) neither subsumes nor is subsumed by (25a).

So we have seen that some feature structures are more specific than others. How do we go about specializing a given feature structure? For example, we might decide that addresses should consist of not just a street number and a street name, but also a city. That is, we might want to merge graph (27a) with (27b) to yield (27c).

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