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<Paper uid="C94-1064">
  <Title>PARSING AS TREE TRAVERSAL</Title>
  <Section position="8" start_page="397" end_page="399" type="concl">
    <SectionTitle>
5 CONCLUSION
</SectionTitle>
    <Paragraph position="0"> In most standard presentations, simple top-down, bottom-up and h'.ft-corner aThis rule is not precis('.ly the same as (.he rules used in BUP since Matsumoto et al. con&gt; pile their rules a lltth! further to take adv~tlltage of the first argument and predicate name indexing used in Prolog.</Paragraph>
    <Paragraph position="1"> parsers are described in terms of pairs c)f op(wations such a.s expand/ma(,c\]l, shift/reduce or sprout/nlatch, l{tlt it is enl, irely unclear wha.(, expa.nding and matching has to do with shifting, reducing or sprouting. By relating parsing (.o tree tri~versal, however, it b(:comes much clearer how these three approac\]ms 1,o parsing rcbd;e to each other.</Paragraph>
    <Paragraph position="2"> This is a natural comparison, since clearly t, he l)OSSiloh: orders in which a tree can be traversed should not dif f(H' frolll the possible orders in which a parse I, ree can be constructed. ~Vhltt's new in this paper, however, is tile idea gha.(, such tree traversal programs could be translated into p~trsers usillg extended (',reibach Nor,ha.1 Form.</Paragraph>
    <Paragraph position="3"> Such a unified approach to parsing is mostly useful simply (,o understand how the different l&gt;arsers are related. It is sm'prising Co see, for examph:, that with partial executiol L the bottom-up and \]el't-cornc.r parsers be('ome, the same.</Paragraph>
    <Paragraph position="4"> The similarity bel;weeu t&gt;ot(,om-u 1) and h:ft-corner pa.rsing ha.s caused a certain all/Ollllt (If (:onI'usion in the literature. l&amp;quot;or example, (,It('. so-calh'd &amp;quot;botton&gt;ui)&amp;quot; chart i)arse.r l)resenl,ed (among other l)laces) in Cazda.r &amp;quot;~ Me.llish \[3\] in fact uses a left-corner strategy. This was pointed out by Wiren \[ll\] but has not receive(l much attention in the litera-I.ure. It is hoped I.ha.1, the unifi('.d approa.ch to parsing l)re.seifix:d h(:re will hel l) 1,o clear u I&gt; ol, her such confusions. Finally, one Inight )nentiol)a co)l-heel.ion to C, ovcrnm('.nt-llinding parsingj a.s presented ill ,Iolmson &amp; Stabhn' \[5\].</Paragraph>
    <Paragraph position="5"> These a.uthors present a generate amd test approa.(:h, in which X-bar strucl, lli'es ~llTe ramlomly generated m~d then tesl, ed agldnst lIB principles. Once (,he logic of the program is expressed in such a ma.uner, cfIi('iency considerations are used in order to fold the testing procedures into the generation procedure.</Paragraph>
    <Paragraph position="6">  One could view the strategy takel~ in this paper as rather similar. Running a tree traversal program in reverse is like randomly generating phrase structure. Then these randomly generated structures are tested against the constraints, i.e., the phrase structure rules. What I have shown here, is that the decision as to where to fold in the constraints is very significant. Folding in the constraints at different positions actually gives completely different parsing strategies.</Paragraph>
  </Section>
class="xml-element"></Paper>