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System Description

Figure 3.1 shows the overall flow of the system when parsing a sentence; a summary of each component is presented in Table 3.1. At the heart of the system is a parser for lexicalized TAGs ([#!schabesjoshi88!#,#!schabes90!#]) which produces all legitimate parses for the sentence. The parser has two phases: Tree Selection and Tree Grafting.
  
Figure 3.1: Overview of XTAG system
\includegraphics[height=3.0in,angle=270]{/mnt/linc/xtag/work/doc/tech-rept/ps/flowchart.eps}


 
Table 3.1: System Summary
Component Details
Morphological Consists of approximately 317,000 inflected items
Analyzer and derived from over 90000 stems.
Morph Database Entries are indexed on the inflected form and return
  the root form, POS, and inflectional information.
POS Tagger Wall Street Journal-trained trigram tagger ([#!kwc88!#])
and Lex Prob extended to output N-best POS sequences
Database ([#!soong90!#]). Decreases the time to parse
  a sentence by an average of 93%.
Syntactic More than 30,000 entries.
Database Each entry consists of: the uninflected form of the word,
  its POS, the list of trees or tree-families associated with
  the word, and a list of feature equations that capture
  lexical idiosyncrasies.
Tree Database 1094 trees, divided into 52 tree families and 218 individual
  trees. Tree families represent subcategorization frames;
  the trees in a tree family would be related to each other
  transformationally in a movement-based approach.
X-Interface Menu-based facility for creating and modifying tree files.
  User controlled parser parameters: parser's start category,
  enable/disable/retry on failure for POS tagger.
  Storage/retrieval facilities for elementary and parsed trees.
  Graphical displays of tree and feature data structures.
  Hand combination of trees by adjunction or substitution
  for grammar development.
  Ability to manually assign POS tag
  and/or Supertag before parsing
 



 
next up previous contents
Next: Tree Selection Up: Overview of the XTAG Previous: Overview of the XTAG
XTAG Project
1998-09-14