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<Paper uid="J79-1052">
  <Title>A COMPUTATIONAL TREATMENT OF COORDINATE CONJUNCTIONS</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
4 . Implementation .................... 20
References ................... 30
Acknowledgment .................... 31
Appendix - The LSP parsing system .......... 32
A COMPUTATIONAL TREATMENT OF COORDINATE CONJUNCTIONS
Carol Raze
</SectionTitle>
    <Paragraph position="0"> One particularly intricate problem in the computer parsing of natural language texts is the complexity introduced into the parsing system by conjunctions. This complexity is due to the richness of conjunctional constructions and to the material implicit in sentences containing conjunctions, The computafional problem can be divided into three parts: (I) generating parse trees which cover the occurrences of conjunction-headed strings in sentences; (11) locating words in the sentences which are repeated implicitly in a &amp;quot;zeroed,&amp;quot; i.e., elided, form in particular positions in conjunction strings; and (III) reconstructing the complete sentences underlying conjunctional occurrences when the application requires such an expansion.</Paragraph>
    <Paragraph position="1"> A generalized algorithmic solution to problem (I) was provided over a decade ago by the New York University Linguistic String Project (LSP) in the framework of linguistic string analysis (Sager e.t al. 1966, Sager 1967, Raze 1967), and similar devices have been described more recently, e.g.,by Woods in the framework of augmented transition networks (Woods 1973). The present paper describes a compukational solution to problem (II), locating the words that are zeroed under conjunction. In this solution, which is based on general properties of conjunctional constructions, a mechanism locates zeroed elements in the conjunction strings and cross-references them with respect to elements in the head construction. Constraints can then be applied to elided conjuncts as though they were expanded, and transformational expansion, which was problem (111) above, can be carried out straightforwardLy by following the pointer's which have been set up.</Paragraph>
    <Paragraph position="2"> The main innovation is called &amp;quot;stacking.&amp;quot; It is a nondeterministic programming device which causes restrictions e., subprograms applying detailed constraints) to be re-executed on conjoined segments wheqever the restriction is invoked on an element which has a conjunct. Since the constraint usually applies to a combination of words in the main string and in the conjunct, the device must ensure that the proper grammatical relation holds between these wards. That is, it treats these separated words as a single linguistic entity, obtaining the effect of a full expansion of the conjunctional occurrence without carrying out the physical rearrangement and copying or the parse tree. The strategy of treating ellipsis in two steps (locating deleted elements and later carrying out the physical expansion) is important for a solution to conjunctions far several reasons. First, it is costly and often fruitless to expand a conjunctional string until one is sure one has a good parse. Second, to get a good parse one has to execute xestrictions on the generated parse tree.including the conjunction subtrees, and this requires locating deleted eldments. And third, the decision as to whether conjunctional constructions should be expanded or not is application specific.</Paragraph>
    <Paragraph position="3"> This artic-le is divided into four sections: 1. The ~efinition of conjunction Strings; 2. Restrictions under Conjunctions; Stacking; 3. Less Common ~eletion Forms; and 4. Implementation. An appendix describes the LSP system.</Paragraph>
  </Section>
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