Carl Pollard

A CENTERING APPROACH TO PRONOUNS

In this paper we present a formalization of the centering approach to modeling attentional structure in discourse and use it as the basis for an algorithm to track discourse context and bind pronouns. As described in [GJW86], the process of centering attention on entities in the discourse gives rise to the intersentential transitional states of continuing, retaining and shifting. We propose an extension to these states which handles some additional cases of multiple ambiguous pronouns. The algorithm has been implemented in an HPSG natural language system which serves as the interface to a database query application.

Structure-Sharing in Lexical Representation

The lexicon now plays a central role in our implementation of a Head-driven Phrase Structure Grammar (HPSG), given the massive relocation into the lexicon of linguistic information that was carried by the phrase structure rules in the old GPSG system. HPSGs grammar contains fewer than twenty (very general) rules; its predecessor required over 350 to achieve roughly the same coverage. This simplification of the grammar is made possible by an enrichment of the structure and content of lexical entries, using both inheritance mechanisms and lexical rules to represent the linguistic information in a general and efficient form. We will argue that our mechanisms for structure-sharing not only provide the ability to express important linguistic generalization about the lexicon, but also make possible an efficient, readily modifiable implementation that we find quite adequate for continuing development of a large natural language system.

Extraposition via Complex Domain Formation

We propose a novel approach to extraposition in German within an alternative conception of syntax in which syntactic structure and linear order are mediated not via encodings of hierarchical relations but instead via order domains. At the heart of our proposal is a new kind of domain formation which affords analyses of extraposition constructions that are linguistically more adequate than those previously suggested in the literature.

A unified theory of scope for quantifiers and wh-phrases

This paper presents an analysis of quantifier and wh-operator scope in terms of a lexicalized theory of quantifier storage, within the framework of Head-Driven Phrase Structure Grammar. The proposed analysis provides an alternative to the derivational approach wherein quantifier scope is determined at a separate level of representation via movement, and shows how scope of quantifiers and wh-phrases (fronted or in situ) can be handled in a unified way in a constraint-based grammar. Lexicalization of quantifier storage offers an account of scope facts in raising and unbounded dependency constructions which have been problematic in an earlier constraint-based approach to quantifier scope.

A Computational Semantics for Natural Language

In the new Head-driven Phrase Structure Grammar (HPSG) language processing system that is currently under development at Hewlett-Packard Laboratories, the Montagovian semantics of the earlier GPSG system (see [Gawron et al. 1982]) is replaced by a radically different approach with a number of distinct advantages. In place of the lambda calculus and standard first-order logic, our medium of conceptual representation is a new logical formalism called NFLT (Neo-Fregean Language of Thought); compositional semantics is effected, not by schematic lambda expressions, but by LISP procedures that operate on NFLT expressions to produce new expressions. NFLT has a number of features that make it well-suited for natural language translations, including predicates of variable arity in which explicitly marked situational roles supercede order-coded argument positions, sortally restricted quantification, a compositional (but nonextensional) semantics that handles causal contexts, and a principled conceptual raising mechanism that we expect to lead to a computationally tractable account of propositional attitudes. The use of semantically compositional LISP procedures in place of lambda-schemas allows us to produce fully reduced translations on the fly, with no need for post-processing. This approach should simplify the task of using semantic information (such as sortal incompatibilities) to eliminate bad parse paths.

PARSING HEAD-DRIVEN PHRASE STRUCTURE GRAMMAR

The Head-driven Phrase Structure Grammar project (HPSG) is an English language database query system under development at Hewlett-Packard Laboratories. Unlike other product-oriented efforts in the natural language understanding field, the HPSG system was designed and implemented by linguists on the basis of recent theoretical developments. But, unlike other implementations of linguistic theories, this system is not a toy, as it deals with a variety of practical problems not covered in the theoretical literature. We believe that this makes the HPSG system unique in its combination of linguistic theory and practical application.The HPSG system differs from its predecessor GPSG, reported on at the 1982 ACL meeting (Gawron et al. ([1982]), in four significant respects: syntax, lexical representation, parsing, and semantics. The paper focuses on parsing issues, but also gives a synopsis of the underlying syntactic formalism.

Categorial Grammar and Phrase Structure Grammar: An Excursion on the Syntax-Semantics Frontier

The notion of a phrase structure grammar first came to the attention of the linguistic community with the publication in 1957 of Syntactic Structures, wherein Noam Chomsky asserted that phrase structure grammars were inadequate for linguistic description. He went on to argue for a model of linguistic structure in which the constituent structure trees produced by a phrase structure grammar were subsequently subjected to operations called transformations that added, deleted, or moved constituents. Successive versions of Chomsky’s transformational grammar came to be accepted as the standard framework for syntactic theory.

Chinese Reflexive Ziji: Syntactic Reflexives vs. Nonsyntactic Reflexives

While recognizing a fundamental distinction between syntactic use of reflexives and nonsyntactic use of reflexives, we propose that this distinction is not necessarily one of lexical ambiguity, contrary to what has been commonly assumed (e.g., Baker (1995)). Instead, we posit just one type of referentially dependent element – reflexives – which avail themselves of two options for being related to their antecedents, namely, syntactic binding and discourse coreference. We focus on Chinese reflexive ziji but will also consider reflexives in American English and British English. Data from these languages indicate that obligatory binding (as stated in Principle A of Chomsky (1986)) is something of a special case and should not be taken as a general model for a cross-linguistic approach to reflexives. We also show that argument structure is relevant to long-distance binding in Chinese, and thus the syntactic binding domain can be defined in terms of relative OBLIQUENESS of grammatical relations rather than a purely tree-configurational relation, e.g., C-COMMAND.

Distinguishing phenogrammar from tectogrammar simplifies the analysis of interrogatives

Oehrle (1994) introduced a categorial grammar architecture in which word order is represented using the terms of a typed λ-calculus and the syntactic type system is based on linear logic. In this paper, we use a variant of this architecture to analyze interrogatives in English and Chinese. We show that separating word order (phenogrammar) and syntactic combinatorics (tectogrammar) in this way brings out the underlying similarities between different question-forming strategies. In particular, the difference between wh extraction (overt movement) and wh in situ (covert movement) turns out to be purely phenogrammatical.

INCLUSION, DISJOINTNESS AND CHOICE: THE LOGIC OF LINGUISTIC CLASSIFICATION

We investigate the logical structure of concepts generated by conjunction and disjunction over a monotonic multiple inheritance network where concept nodes represent linguistic categories and links indicate basic inclusion (ISA) and disjointness (ISNOTA) relations. We model the distinction between primitive and defined concepts as well as between closed-and open-world reasoning. We apply our logical analysis to the sort inheritance and unification system of HPSG and also to classification in systemic choice systems.