Briscoe, E., C. Grover, B. Boguraev and J. Carroll (1987) `Feature defaults, propagation and reentrancy'. Workshop on Natural Language Processing, Unification and Grammar Formalisms. In E. Klein and J. van Bentham (eds.), Categories, Polymorphism and Unification, Centre for Cognitive Science, University of Edinburgh. 19-34.
We have provided a computationally tractable interpretation of GPSG and developed a grammar of English with substantial coverage within this revised framework. Our reinterpretation of GPSG dispenses with the simultaneous projection of immediate dominance (ID) rules to local trees and replaces this with a procedurally-defined expansion of the metagrammar into a (large) set of phrase structure rules whose categories are feature sets which are matched by unification at parse time. The paper will focus on the way we handle feature defaults and feature propagation within this unification-based formalism. We present a notation in which feature propagation principles (such as the Head Feature Convention) can be defined in terms of ID rule patterns. We treat feature defaults in a very similar fashion, avoiding the complex interpretation of GPSG defaults by explicitly stating the (ID rule) environment in which the default will apply. Finally, we outline the current state of the grammar of English under development in this formalism and argue that a fully reentrant feature system is not required in this grammar, which covers constructions analysed using category-valued features in GPSG.