Tree-adjoining grammars (TAG) have been proposed as a formalism for generation based on the intuition that the extended domain of syntactic locality that TAGs provide should aid in localizing semantic dependencies as well, in turn serving as an aid to generation from semantic representations. We demonstrate that this intuition can be made concrete by using the formalism of synchronous tree-adjoining grammars. The use of synchronous TAGs for generation provides solutions to several problems with previous approaches to TAG generation. Furthermore, the semantic monotonicity requirement previously advocated for generation gram- mars as a computational aid is seen to be an inherent property of synchronous TAGs.

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This paper describes a new interpretation of Tree Adjoining Grammars (TAG) that allows the embedding of TAG in the unification framework in a manner consistent with the declarative approach taken in this framework. In the new interpretation we present in this paper, the objects manipulated by a TAG are considered to be descriptions of trees. This is in contrast to the traditional view that in a TAG the composition operations of adjoining and substitution combine trees. Borrowing ideas from Description Theory, we propose quasi-trees as a means to represent partial descriptions of trees. Using quasi-trees, we are able to justify the definition of feature structure-based Tree Adjoining Grammars (FTAG) that was first given in Vijay-Shanker (1987) and Vijay-Shanker and Joshi (1988). In the definition of the FTAG formalism given here, we argue that a grammar manipulates descriptions of trees (i.e., quasi-trees); whereas the structures derived by a grammar are trees that are obtained by taking the minimal readings of such descriptions. We then build on and refine the earlier version of FTAG, give examples that illustrate the usefulness of embedding TAG in the unification framework, and present a logical formulation (and its associated semantics) of FTA G that shows the separation between descriptions of well-formed structures and the actual structures that are derived, a theme that is central to this work. Finally, we discuss some questions that are raised by our new interpretation of the TAG formalism: questions dealing with the nature and definition of the adjoining operation (in contrast to substitution), its relation to multi-component adjoining, and the distinctions between auxiliary and initial structures.

There is currently considerable interest among computational linguists in grammatical formalisms with highly restricted generative power. This paper concerns the relationship between the class of string languages generated by several such formalisms viz. Combinatory Categorial Grammars, Head Grammars, Linear Indexed Grammars and Tree Adjoining Grammars. Each of these formalisms is known to generate a larger class of languages than Context-Free Grammars. The four formalisms under consideration were developed independently and appear superficially to be quite different from one another. The result presented in this paper is that all four of the formalisms under consideration generate exactly the same class of string languages. 1 Introduction There is currently considerable interest among computational linguists in grammatical formalisms with highly restricted generative power. This is based on the argument that a grammar formalism should not merely be viewed as a notation, but as part o...

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The strict competence hypothesis has sparked a small dialogue among several researchers attempting to understand its ramifications for human sentence processing and incremental interpretation in particular. In this paper, we review the dialogue, reconstructing the arguments in an attempt to make them more uniform and crisper, and provide our own analyses of certain of the issues that arise. We argue that strict competence, because it requires a synchronous computation mechanism, may actually lead to more complex, rather than simpler, models of incremental interpretation. Asynchronous computation, which is arguably both psychologically more plausible and conceptually more basic, allows for incremental interpretation to fall out naturally, without additional machinery for interpreting partial constituents. We show that this is true regardless of whether the presumed interpretation mechanism is top-down or bottomup, contra previous conclusions in the literature, and propose a particular i...

al., 1995) point out the differences between TAG derivation structures and semantic or predicate- argument dependencies, and Joshi and VijayShanker (Joshi and Vijay-Shanker, 1999) de- scribe a monotonic compositional semantics based on attachment order that represents the desired dependencies of a derivation without underspecifying predicate-argument relationships at any stage. In this paper, we apply the Joshi and Vijay-Shanker conception of compositional semantics to the problem of preserving semantic dependencies in Synchronous TAG translation (Shieber and Schabes, 1990; Abeill6 et al., 1990). In particular, we describe an algorithm to obtain the semantic dependencies on a TAG parse forest and construct a target derivation forest with isomorphic or locally non-isomorphic dependencies in O(n 7) time.

Several methods are known for parsing languages generated by Tree Adjoining Grammars (TAGs) in O(n^6) worst case running time. In this paper we investigate which restrictions on TAGs and TAG derivations are needed in order to lower this O(n^6) time complexity, without in- troducing large runtime constants, and without losing any of the generatire power needed to capture the syntactic constructions in natural language that can be handled by unrestricted TAGs. In particular, we describe an algorithm for parsing a strict subclass of TAG in (n^5), and attempt to show that this subclass retains enough generafive power to make it useful in the general case.

this paper. At a higher level of description, the grammar rules and principles that are implicit in the form of the lexicon would be stated explicitly. For example, there are principles which govern which trees are grouped together into tree families, and rules which describe the relations between structure types across tree families (see subsection 2.1 for a discussion of tree families.) The information explicitly provided in this more abstract representation of the grammar may be thought of as an interpretation of the data in the lower-level representation.