John T. Maxwell

Parsing the Wall Street Journal using a Lexical-Functional Grammar and Discriminative Estimation Techniques

We present a stochastic parsing system consisting of a Lexical-Functional Grammar (LFG), a constraint-based parser and a stochastic disambiguation model. We report on the results of applying this system to parsing the UPenn Wall Street Journal (WSJ) treebank. The model combines full and partial parsing techniques to reach full grammar coverage on unseen data. The treebank annotations are used to provide partially labeled data for discriminative statistical estimation using exponential models. Disambiguation performance is evaluated by measuring matches of predicate-argument relations on two distinct test sets. On a gold standard of manually annotated f-structures for a subset of the WSJ treebank, this evaluation reaches 79% F-score. An evaluation on a gold standard of dependency relations for Brown corpus data achieves 76% F-score.

Formal issues in lexical-functional grammar

Preface Part I. Formal Architecture: 1. The formal architecture of Lexical Functional Grammar Ronald M. Kaplan 2. Lexical Functional Grammar: a formal system for grammatical representation Ronald M. Kaplan and Joan Bresnan Part II. Nonlocal Dependencies: 3. Long-distance dependencies, constituent structure, and functional uncertainty Ronald M. Kaplan and Annie Zaenen 4. Modeling syntactic constrants on anaphoric binding Mary Dalrymple, John T. Maxwell III, and Annie Zaenen 5. An algorithm for functional uncertainty Ronald M. Kaplan and John T. Maxwell III 6. Constituent coordination in lexical functional grammar Ronald M. Kaplan and John T. Maxwell III Part III. Word Order: 7. Formal devices for linguisic generalizations: West Germanic world order in lexical functional grammar Annie Zaenen and Ronald M. Kaplan 8. Linear order, syntactic rank, and empty categories: on weak crossover Joan Bresnan Part IV. Semantics and Translation: 9. Projections and semantic description in lexical-functional grammar Per-Kristian Halvorsen and Ronald M. Kaplan 10. Situation semantics and semantic interpretation in constraint-based grammars Per-Kristian Halvorsen 11. Translation by structural correspondences Ronald M. Kaplan, Klaus Netter, Jurgen Wedekind and Annie Zaenen Part V. Mathematical and Computational Issues: 12. Three seductions of computational psycholinguistics Ronald M. Kaplan 13. Logic and feature structures Mark Johnson 14. A method for disjunctive constraint satisfaction John T. Maxwell III and Ronald M. Kaplan 15. The interface between phrasal and functional constraints John T. Maxwell III and Ronald M. Kaplan.

A Method for Disjunctive Constraint Satisfaction

A distinctive property of many current grammatical formalisms is their use of feature equality constraints to express a wide variety of grammatical dependencies. Lexical-Functional Grammar (Kaplan & Bresnan, 1982), Head-Driven Phrase-Structure Grammar (Pollard & Sag, 1987), PATR (Karttunen, 1986a), FUG (Kay, 1979, 1985), and the various forms of categorial unification grammar (Karttunen, 1986b; Uszkoreit, 1986; Zeevat, Klein, & Calder, 1987) all require an analysis of a sentence to satisfy a collection of feature constraints in addition to a set of conditions on the arrangement of words and phrases. Conjunctions of equality constraints can be quickly solved by standard unification algorithms, so they in themselves do not present a computational problem. However, the equality constraints derived for typical sentences are not merely conjoined together in a form that unification algorithms can deal with directly. Rather, they are embedded as primitive elements in complex disjunctive formulas. For some formalisms, these disjunctions arise from explicit disjunction operators that the constraint language provides for (e.g., LFG) while for others disjunctive constraints are derived from the application of alternative phrase structure rules (e.g., PATR). In either case, disjunctive specifications help to simplify the statement of grammatical possibilities. Alternatives expressed locally within individual rules and lexical entries can appeal to more general disjunctive processing mechanisms to resolve their global interactions.

An algorithm for functional uncertainty

The formal device of functional uncertainty has been introduced into linguistic theory as a means of characterizing long-distance dependencies alternative to conventional phrase-structure based approaches. In this paper we briefly outline the uncertainty concept, and then present an algorithm for determining the satisfiability of acyclic grammatical descriptions containing uncertainty expressions and for synthesizing the grammatically relevant solutions to those descriptions.

Grammatical Machine Translation

We present an approach to statistical machine translation that combines ideas from phrase-based SMT and traditional grammar-based MT. Our system incorporates the concept of multi-word translation units into transfer of dependency structure snippets, and models and trains statistical components according to state-of-the-art SMT systems. Compliant with classical transfer-based MT, target dependency structure snippets are input to a grammar-based generator. An experimental evaluation shows that the incorporation of a grammar-based generator into an SMT framework provides improved grammaticality while achieving state-of-the-art quality on in-coverage examples, suggesting a possible hybrid framework.

Constituent coordination in Lexical-Functional Grammar

This paper outlines a theory of constituent coordination for Lexical-Functional Grammar. On this theory LFGs flat, unstructured sets are used as the functional representation of coordinate constructions. Function-application is extended to sets by treating a set formally as the generalization of its functional elements. This causes properties attributed externally to a coordinate structure to be uniformly distributed across its elements, without requiring additional grammatical specifications.

Adapting existing grammars: the XLE experience

We report on the XLE parser and grammar development platform (Maxwell and Kaplan, 1993) and describe how a basic Lexical Functional Grammar for English has been adapted to two different corpora (newspaper text and copier repair tips).

Speeding up LFG Parsing Using C-Structure Pruning

In this paper we present a method for greatly reducing parse times in LFG parsing, while at the same time maintaining parse accuracy. We evaluate the methodology on data from English, German and Norwegian and show that the same patterns hold across languages. We achieve a speedup of 67% on the English data and 49% on the German data. On a small amount of data for Norwegian, we achieve a speedup of 40%, although with more training data we expect this figure to increase.

Pruning the Search Space of a Hand-Crafted Parsing System with a Probabilistic Parser

The demand for deep linguistic analysis for huge volumes of data means that it is increasingly important that the time taken to parse such data is minimized. In the XLE parsing model which is a hand-crafted, unification-based parsing system, most of the time is spent on unification, searching for valid f-structures (dependency attribute-value matrices) within the space of the many valid c-structures (phrase structure trees). We carried out an experiment to determine whether pruning the search space at an earlier stage of the parsing process results in an improvement in the overall time taken to parse, while maintaining the quality of the f-structures produced. We retrained a state-of-the-art probabilistic parser and used it to pre-bracket input to the XLE, constraining the valid c-structure space for each sentence. We evaluated against the PARC 700 Dependency Bank and show that it is possible to decrease the time taken to parse by ~18% while maintaining accuracy.

Modeling syntactic constraints on anaphoric binding

Syntactic constraints on antecedent-anaphor relations can be stated within the theory of Lexical Functional Grammar (henceforth LFG) through the use of functional uncertainty (Kaplan and Maxwell 1988; Halvorsen and Kaplan 1988; Kaplan and Zaenen 1989). In the following, we summarize the general characteristics of syntactic constraints on anaphoric binding. Next, we describe a variation of functional uncertainty called inside-out functional uncertainty and show how it can be used to model anaphoric binding. Finally, we discuss some binding constraints claimed to hold in natural language to exemplify the mechanism. We limit our attention throughout to coreference possibilities between definite antecedents and anaphoric elements and ignore interactions with quantifiers. We also limit our discussion to intrasentential relations.