Maximal Consistent Subsets

Default unification operations combine strict information with information from one or more defeasible feature structures. Many such operations require finding the maximal subsets of a set of atomic constraints which are consistent with each other and with the strict feature structure, where a subset is maximally consistent with respect to the subsumption ordering if no constraint can be added to it without creating an inconsistency. While this problem is NP-complete, there are a number of heuristic optimizations that can be used to substantially reduce the size of the search space. In this paper, we propose a novel optimization, leaf pruning, which in some cases yields an improvement in running time of several orders of magnitude over previously described algorithms. This makes default unification efficient enough to be practical for a wide range of problems and applications. 1

Translating the XTAG English Grammar to HPSG

Introduction We describe the development of XHPSG, a large-scale English grammar in the HPSG formalism translated from the XTAG grammar (The XTAG Research Group, 1995). Our goal is to obtain a large-scale, linguistically sound grammar for our HPSG parser (Makino et al., 1998) with a relatively small workload. For this purpose, we try to make an HPSG grammar equivalent to the XTAG grammar in the strong sense where we preserve the structures and the linguistic analysis of the XTAG grammar. To guarantee the equivalence of the XHPSG and XTAG grammars, the following conditions must be satisfied: 1) An XTAG elementary tree is translated to an XHPSG lexical item that translates back to the original elementary tree by applying the schemata and principles; 2) No XHPSG lexical item translates back to a tree other than the original XTAG elementary tree; 3) Substitution and adjunction allowed in the original grammar, and no other opeations, are simulated in the XHPSG parsing. We not on