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<Paper uid="A92-1030">
  <Title>XTAG - A Graphical Workbench for Developing Tree-Adjoining Grammars*</Title>
  <Section position="2" start_page="0" end_page="223" type="intro">
    <SectionTitle>
1 Introduction
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    <Paragraph position="0"> Tree-adjoining grammar (TAG) \[Joshi et al., 1975; Joshi, 1985; Joshi, 1987\] and its lexicalized variant \[Schabes et al., 1988; Schabes, 1990; Joshi and Schabes, 1991\] are tree-rewriting systems in which the syntactic properties of words are encoded as tree structured-objects of extended size. TAG trees can be combined with adjoining and substitution to form new derived trees. 1 Tree-adjoining grammar differs from more traditional tree-generating systems such as context-free grammar in  two ways: 1. The objects combined in a tree-adjoining grammar (by adjoining and substitution) are trees and not strings. In this approach, the lexicon associates with a word the entire structure it selects (as shown in Figure 1) and not just a (non-terminal) symbol as in context-free grammars.</Paragraph>
    <Paragraph position="1"> 2. Unlike string-based systems such as context-free  grammars, two objects are built when trees are combined: the resulting tree (the derived tree) and its derivational history (the derivation tree). 2 These two unique characteristics of tree-adjoining grammars, the elementary objects found in the lexicon (extended trees) and the distinction between derived tree and its derivational history (also a tree), require a specially crafted interface in which the perspective must be shifted from a string-based to a tree-based system. 1We assume familiarity throughout the paper with the definition of TAGs. See the introduction by Joshi \[1987\] for an introduction to tree-adjoining grammar. We refer the reader to Joshi \[1985\], Joshi \[1987\], Kroch and Joshi \[1985\], Abeill~ et al. \[1990a\], Abeill~ \[1988\] and to Joshi and Schabes \[1991\] for more information on the linguistic characteristics of TAG such as its lexicalization and factoring recursion out of dependencies.</Paragraph>
    <Paragraph position="2"> 2The TAG derivation tree is the basis for semantic interpretation \[Shieber and Schabes, 1990b\], generation \[Shieber and Schabes, 1991\] and machine translation \[Abeill~ et al., 1990b\] since the information given in this data-structure is richer than the one found in the derived tree. Furthermore, it is at the level of the derivation tree that ambiguity must be defined.</Paragraph>
    <Paragraph position="3">  grammar lexicon XTA G provides such a graphical interface in which the elementary objects are trees (or tree sets) and not symbols (or strings of symbols).</Paragraph>
    <Paragraph position="4"> Skeletons of such workbenches have been previously realized on Symbolics machines \[Schabes, 1989; Schifferer, 1988\]. Although they provided some insights on the architectural design of a TAG workbench, they were never expanded to a full fledged natural language environment because of inherent limitations (such as their lack of portability).</Paragraph>
    <Paragraph position="5"> XTAG runs under Common Lisp \[Steele, 1990\] and it uses the Common LISP X Interface (CLX) to access the graphical primitives defined by the Xll protocol. XTAG is portable across machines and Common Lisp compilers. The kernel of XTA G is a predictive left to right parser for unification-based tree-adjoining grammar \[Schabes, 1991\]. The system includes the following components and features: * Graphical edition of trees. The graphical display of a tree is the only representation of a tree accessible to the user. Some of the operations that can be performed graphically on trees are:  - Add and edit nodes.</Paragraph>
    <Paragraph position="6"> - Copy, paste, move or delete subtrees.</Paragraph>
    <Paragraph position="7"> - Combine two trees with adjunction or substitution. These operations keep track of the derivational history and update attributes stated in form of feature structures as defined in the framework of unification-based tree-adjoining grammar \[Vijay-Shanker and Joshi, 1988\].</Paragraph>
    <Paragraph position="8"> - View the derivational history of a derived tree and its components (elementary trees).</Paragraph>
    <Paragraph position="9"> * A tree display module for efficient and aesthetic formatting of a tree based on a new tree display algo null rithm \[Chalnick, 1989\]. The algorithm is an improvement of the ones developed by R.eingold and Tolford \[1981\] and, Lee \[1987\]. It guarantees in linear time that tress which are structural mirror images of on another are drawn such that their displays are reflections of one another while achieving minimum width of the tree.</Paragraph>
    <Paragraph position="10"> * Capabilities for grouping trees into sets which can be linked to a file. This is particularly useful since lexicalized TAGs organize trees into tree-families</Paragraph>
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