The First Bug Report

1. Introduction The Budapest Unification Grammar (BUG) system described in this paper is a system for generating natural language parsers from feature-structure based grammatical descriptions (graamnars). In the current version, source grammars are limited to the context-free phrase structure grammar format. BuG compiles source grmnmars into automata, which it can then use for parsing input strings. BUG differs from other unification-based grammar-writing tools in two major respects as well as in a number of minor ways. One major difference is that nu(~ uses feature geometries. The feature geometry is a (recursive) definition of well-formed feature structures, which must be specified in the source grammar. The other major difference is that BUG uses a built-in performance restriction, called tile string completion limit (SCL). Using the string completion limit, we can limit the generative power of a context-free grammar to regular languages. The paper focuses on these two innovations as well as a third feature of huG, which is the separation of the structural description (SD, conditions of application) from the structural change (SC, effect of application) in source rules. Tile term feature geometry is taken from gen-erative phonology, where it was introduced by Clements (1985). A feature geometry determines what feature structures are allowed by specifying what (complex or atomic) values each path in a feature structure can have. In this way, a feature geometry expresses certain kinds of feature co-occurrence restrictions (FCRs, Gazdar et al., 1985), namely, those FCRs that are local in the sense that they can be formulated in terms of path continuation restrictions. For example, we can incorporate the FCIt [TENSE ~-PAST] z:~ [FINITE] in a geometry by making TENSE a sub-feature of only FINITE (and PAST a possible value of TENSE). On the otlmr hand, we cannot encode a global FCR like [SUBJ DEF : +]-:~ [INDIR_OBJ NUMBER : PLURAL]. Also, we cannot encode a global FCR such as [TENSE = PAST] ~ [AGREEMENT] unless we make TENSE a sub-feature of AGREEMENT alone. This is important because allowing arbitrary or global constraints on wen-fornmd feature structures leads to undecidable systems if coupled with structure sharing (Blackburn and Spaan, 1991). Our feature geometries, just like the ones used ill phonology, specify whether or not the continuations of a given path are pairwise incmnpatible. For example, the attributes FINITE and NON-FINITE can be made incompatible continuations of the attribute VERB_FORM. As a result, in any actual feature structure …