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<Paper uid="P81-1036">
  <Title>Chart Parsing and Rule Schemata in PSG</Title>
  <Section position="1" start_page="0" end_page="0" type="intro">
    <SectionTitle>
INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> MCHART is a flexible, modular chart parsing framework I have been developing (in Lisp) at Edinburgh, whose initial design characteristics were largely determined by pedagogical needs.</Paragraph>
    <Paragraph position="1"> PSG is a gr---n-tical theory developed by Gerald Gazdar at Sussex, in collaboration with others in both the US and Britain, most notably Ivan Sag, Geoff Pull,--, and Ewan Klein. It is a notationally rich context free phrase structure grumm~r, incorporating meta-rules and rule schemata to capture generalisations. (Gazdar 198Oa, 1980b, 1981; Gazdar &amp; Sag 1980; Gazdar, Sag, Pullum &amp; Klein to appear) In this paper I want to describe how I have used MCHART in beginning to construct a parser for gr-mm-rs expressed in PSG, and how aspects of the chart parsing approach in general and MCHART in particular have made it easy to acco~mmodate two significant aspects of PSG: rule schemata involving variables over categories; and compound category symbols (&amp;quot;slash&amp;quot; categories). To do this I will briefly introduce the basic ideas of chart parsing; describe the salient aspects of MEHART; give an overview of PSG; and finally present the interesting aspects of the parser I am building for PSG using MCHART. Limitations of space, time, and will mean that all of these sections will be brief and sketchy - I hope to produce a much expanded version at a later date.</Paragraph>
    <Paragraph position="2"> I. Chart Parsing The chart parsing idea was originally conceived of by Martin Kay, and subsequently developed and refined by him and Rot Kaplan (Kay 1973, 1977, 1980; Kaplan 1972, 1973a, 19735). The basic idea builds on the device known as a well formed substring table, and transforms it from a passive repository of achieved results into an active parsing agent. A well formed substring table can be considered as a directed graph, with each edge representing a node in the analysis of a string.</Paragraph>
    <Paragraph position="3"> Before any parsing has occurred, all the nodes are (pre)terminal, as in Figure I.</Paragraph>
    <Paragraph position="4"> Figure I. Kim saw the child with he lass$ N V O N P D N Non-terminal nodes discovered in the course of parsing, by whatever method, are recorded in the WFST by the addition of edges to the graph. For example in Figure 2 we see the edges which might have been added in a parsing of the sentence given in Figure I.</Paragraph>
  </Section>
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