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<Paper uid="P84-1073">
  <Title>LR Pa rse rs For Natural Languages,</Title>
  <Section position="2" start_page="0" end_page="354" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> LR parsers\[I, 2\] have been developed originally for programming language of compilers. An LR parser is a shift-reduce parser which is detenninistically guided by a par.~it~g table indicating what action should be taken next. The parsing table can be obtained automatically from a context-free phrase structure grammar, using an algorithm first developed by DeRemer \[5, 6\]. We do not describe the algorithm here, reffering the render to Chapter 6 in Aho and UIIman \[4\]. The LR parsers have seldom been used for Natural Language Processing  probably because: 1. It has been thought that natural languages are not context-free, whereas LR parsers can deal only with context-free languages.</Paragraph>
    <Paragraph position="1"> 2. Natural languages are ambiguous, while standard LR  parsers can not handle ambi~juous languages.</Paragraph>
    <Paragraph position="2"> The recent literature\[8\] shows that the belief &amp;quot;natural languages are not context-free&amp;quot; is not necessarily true, and there is no reason for us to give up the context-freedom of natural languages. We (to not discuss on this matter further, considering the fact that even if natural languages are not context-free, a fairly comprehensive grammar for a subset of natural language suflicient for practical systems can be written in context.free phrase structure. lhtJ.% our main concern is how to cope with the ambiguity of natural languages, and this concern is addressed in the fallowing section.</Paragraph>
    <Paragraph position="3"> 2 LR parsers and Ambiguous Grammars If a given grammar is ambiguous? we cannot have a parsing table in which ~ve~y entry is uniquely defined; at lea~t one entry of it~ parsing table is inulliply defined. It has been thought that, for LR pa~sers, nndtiple entries are fatal because they make deterministic parsing no longer po~$ible.</Paragraph>
    <Paragraph position="4"> Aho et. al. \[3\] and Shieber\[121 coped with this ambiguity problem by statically 3 selecting one desired action out of multiple actions, and thus converting n=ulliply-defined entries into uniquely-defined ones.With this approach, every input sentence has no more than one parse tree. This fact is desirable for progralnming languages.</Paragraph>
    <Paragraph position="5"> For natural languages, however, it is sometimes necessary for a parser to produce more than one parse tree. For example, consider the following short story.</Paragraph>
    <Paragraph position="6"> I saw the man with a telescope.</Paragraph>
    <Paragraph position="7"> He should have bought it at the department store.</Paragraph>
    <Paragraph position="8"> When the first sentence is read, there is absolutely no way to resolve the ambiguity 4 at that time. The only action the system can take is to produce two parse trees and store them somewhere for later disambiguation.</Paragraph>
    <Paragraph position="9"> In contrast with Aho et. al. and Shieber, our approach is to extend LR parsers so that they can handle multiple entries and produce more than one parse tree if needed. We call the extended LR parsers MLR parsers.</Paragraph>
    <Paragraph position="10"> ll'his rP.~i:i'C/l'Ctl was -~pon~oled by the Df.'ieose Advanced Research Projects Agency (DOD), ARPA Older No. 3597, munitoled hy lhe Air Foi'r:e Avionics Lot)oratory Under C, uolracl F3:)(~15 81 K-t539. The views and con,.;lusion$ conl,lii~cd i=1 lhi.~; (lo=;unlq;nt a~i.~ tho'.;e () |tt1~.! ;iu|hor.~; alld should not be illlerpreted as n:pre.-',enling the official p(':licie:;, c, ilher expressed or implied, of the Defense</Paragraph>
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