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<Paper uid="C90-2052">
  <Title>Reversible Unification Based Machm. Franslatlon</Title>
  <Section position="1" start_page="0" end_page="0" type="abstr">
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Abstract
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    <Paragraph position="0"> \[n this paper it will be shown how unification grammars can be used to build a reversible machine translation system.</Paragraph>
    <Paragraph position="1"> Unification grammars are often used to define the relation between strings and meaning representations in a declarative way. Such grammars are sometimes used in a bidirecLional way, thus the same grammar is used for both parsing and generation, in this paper 1 will show how ~.o use bidirectional unification grammars to dethle reversible relations between language dependent meaning representations. Furthermore it is shown how to obtain a completely reversible MT system using a series of (bidirectional) unification grammars.</Paragraph>
    <Paragraph position="2"> l Introduction The notion of a rew:rsible MT system was first expressed by Landsbergen \[11\]. Such a system will in principle produce a set of possible translations, by employing linguistic knowledge only. Choosing the best iranslation from the set. of linguistically possible translations will rsually require other sources of knowledge, either incorporated in the systenl or provided (interactively) by the. user. The relation 'possible translation' is symmetric whereas the relation 'best translation' is not. Thus an MT system may consist of a reversible core, implementi~:g the symmetric relation ~possible translation', and additional components (not necessarily reversible) to select tile best translation.</Paragraph>
    <Paragraph position="3"> Not only is it pos.siblc to build reversible (modules of) MT systems; it has also been claimed that reversible systems are preferable. For example Isabelle \[6\] clahns that reversible MT systems are to be preferred to oth(:rs be(:~nlse in reversible MT systems a better understanding of the translation re.lation is achieved; such systems will eventually exhibit better practicM performance. Moreover, the arguments in favour of using bidirectional grammars in NL1 ), such as those given in \[1, 8\] carry over to translation as well.</Paragraph>
    <Paragraph position="4"> Because o\[ tile declarative nature of unification- and logic grammar formMisms grammars written in these formalisms are increasingly used in a bidirectional way, thus the sa.me grammar is used for both parsing and generation. Some recent developments are reported in \[3, 24, 16, 21, 2, 18, 19, 22, 20\].</Paragraph>
    <Paragraph position="5"> In this paper I will show how to use such bidirectional unification grammars to build a completely reversible, multilingual, MT system. For each language there is a unification grammar that defines a reversible relation between strings and language dependent lneaning representations (logical forms ). Moreover, for each language pair (or set of languages) there is a unification grammar that defines a reversible relation between such language dependent logicM forms. Translation is thus detined by a series of three unification granrmars.</Paragraph>
    <Paragraph position="6"> A specific version of the system that is described here is implemented as the (:ore of the experiments! MiMo2 translation system \[23\]. This system aims at translating international news items on felctext. Apart from unification grammars the system rises a bidirectional two-level orthography component. Langu age de.pendent meanings are represented as simple predicate argument structures with some extra labels indicating 'universal' meaning such as tense and aspect. The cur.rent system (November 1989) includes grammars for Dutch, Spanish and English.</Paragraph>
    <Paragraph position="7"> The paper is set up as follows. In section 2, I will give some examples that show how bidirectional unitication grammars can be used t.o define relation~ be tween logical forms of diiferent languages. In section 3, reversibility is detined in terms of symmetry and con&gt; patability. Possible approaches to obtedu reversibility are discussed. In section 4, I wilt compare the current approach with some other approaches in tile unification based translation paradigm and discuss some problems and future directions.</Paragraph>
  </Section>
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