Order Independent and Persistent Typed Default Uniication

We deene an order independent version of default uniication on typed feature structures. The operation is one where default information in a feature structure typed with a more speciic type, will override default information in a feature structure typed with a more general type, where speciicity is deened by the subtyping relation in the type hierarchy. The operation is also able to handle feature structures where reentrancies are default. We provide a formal semantics, prove order independence and demonstrate the utility of this version of default uniication in several linguistic applications. First, we show how it can be used to deene multiple orthogonal default inheritance in the lexicon in a fully declarative fashion. Secondly, we show how default lexical speciications (introduced via default lexical inheritance) can be made to usefully`persist beyond the lexicon' and interact with syntagmatic rules. Finally, we outline how persistent default uniication might underpin default feature propagation principles and a more restrictive and constraint-based approach to lexical rules.