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<Paper uid="H92-1051">
  <Title>Interaction between Structural Changes in Machine Translation</Title>
  <Section position="4" start_page="255" end_page="255" type="metho">
    <SectionTitle>
3. Transfer
</SectionTitle>
    <Paragraph position="0"> We follow the standard division of a machine translation system into stages of analysis, transfer, and generation. It is a requirement for the transfer description that it should state all and only the equivalence relationships between expressions of a pair of languages. It should contain no general linguistic information, either universal or specific to any single language: these types of information belong in the formalism and content, respectively, of the descriptions of individual languages.</Paragraph>
    <Paragraph position="1"> In fact most of our transfer descriptions consist of direct equivalences between predicates or groups of predicates of the language pair.</Paragraph>
    <Paragraph position="2"> A transfer rule shows a particular translation equivalence between two languages. It consists of three parts: a pair of logical forms, one for each language, representing the equivalence, and a third logical form giving the conditions under which the equivalence is applicable. We call these the equivalence pair and the condition respectively.</Paragraph>
    <Paragraph position="3"> Two ~ logical forms form an equivalence pair if the natural language expressions they represent have overlapping denotations. The rule can therefore be used for translating in either direction: the two logical forms of the equivalence pair are always translationally equivalent if the condition is satisfied. The logical forms of the equivalence pair will be indistinguishable from logical forms of the respective languages, using the same predicates and relations. The logical forms of the condition use meta-predicates which allow reference to to the logical form of the current local translation unit (sentence or clause) and to the linguistic context. In practice, most transfer rules have no condition; they consist just of an equivalence pair. Some examples of rules follow:</Paragraph>
    <Paragraph position="5"> The following is the basic definition of transfer between two texts (sentences, clauses, or whatever): A source logical form and a target logical form are correctly transferred if the terms of each can be divided into non-overlapping subsets such that the source subsets can be placed into one-to-one correspondence with the target subsets by each corresponding pair being 'unified' with the two halves of the equivalence pair of a transfer rule, and if the accumulated conditions of the transfer rules used are satisfied. ('Unification' in this definition is rather similar to graph unification).</Paragraph>
  </Section>
  <Section position="5" start_page="255" end_page="259" type="metho">
    <SectionTitle>
4. 'Complex' Transfer
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="255" end_page="257" type="sub_section">
      <SectionTitle>
4.1. Types of Complex Transfer
</SectionTitle>
      <Paragraph position="0"> Lindop &amp; Tsujii (1991) list and discuss a variety of examples that are always problematic for machine translation systems. We suggest that what makes these examples difficult is that different dependency structures are associated with otherwise synonymous lexical items. We break the problem down into four subparts.</Paragraph>
      <Paragraph position="2"> Argument-swiiching as in the translation of the German Der Wagen gef~llt mir -- I like the car The car pleases me</Paragraph>
      <Paragraph position="4"> In argument-switching, the relations between the lexical item and its dependents are not translated standardly.</Paragraph>
      <Paragraph position="5"> Although the German nominative normally corresponds to an English subject, it must in this example be translated as a dative.</Paragraph>
      <Paragraph position="6"> * ii) Head-switching as in the translation of German Hans schwimmt gern -- John likes swimming John swims gladly like(L) ~ oDj(L,S), , gem(L) ~ subj(L,S) iffshift(L,S) The German sentence is a description of a situation to do with swimming; the English is a description of a situation to do with liking. The shift predicate is explained below.</Paragraph>
      <Paragraph position="7">  * iii) Decomposition as in the translation of the Japanese John-ga zisatusita -- John committed suicide zisatusuru(E) , , commit(E) &amp; suicide(S) &amp; obj(E,S) Here the single Japanese word zisatusuru is translated as the English phrase to commit suicide. Some types of decomposition can give rise to special problems when there are modifiers or dependents involved. These are discussed in detail by Tsujii et al. (1992). Shortage of space and the variety and complexity of examples prevents their discussion here.</Paragraph>
      <Paragraph position="8"> * iv) Raising as in the translation of the Welsh Fe ddigwydd fod Si6n yma happens be John here John happens to be here</Paragraph>
      <Paragraph position="10"> In the English, the subject of happen is John, but in the Welsh, the subject of digwydd ('happen') is the situation of John's being here.</Paragraph>
      <Paragraph position="11"> The predicate shift is introduced to define a translation equivalence where certain dependencies are differently arranged in the target language, as compared to the source language. It can be understood in terms of semantic type-shifting -- the types are differently distributed in the source- and target-language logical forms. Shift(H,E) means that in any instance of subj(H,X} or P(X,H) (where P stands for any relation) in the logical form representing the English text, the H corresponds to an E in the logical form representing the other language.</Paragraph>
      <Paragraph position="12"> The following example shows how the example under (ii) above would work in practice.</Paragraph>
      <Paragraph position="13"> Ich weiB Hans schwimmt gern- I know John likes swimming null</Paragraph>
      <Paragraph position="15"> The columns of the table show the German source logical form, the English logical form as it would be without shift, and the actual English target logical form. The horizontal divisions of the table represent the fields of operation of separate transfer rules.</Paragraph>
      <Paragraph position="16"> 4.2. Interaction between types of complex transfer When several of these syntactic changes are combined in one example, the logical form resulting from transfer tend to be incoherent. The following examples are types on which most, if not all, existing machine translation systems will fail, as will the mechanism proposed here so far.</Paragraph>
      <Paragraph position="17"> English - Welsh; raising with argument-switching: John happens to like swimming fe ddigwydd fod yn dda gan John nofio happens be nice by John swim  John happens to like swimming (also 'John likes happening to swim 3 There are two separate causes of difficulty in these examples. Firstly, the long-distance movement caused by raising causes problems n the transfer of structures which have been the target of raising. The simplest way to get round the problem is to require syntactically-deleted arguments to be marked explicitly on all the verbs of which they are logical arguments, treating the phenomenon syntactically as an example of lexically-triggered extraction, on a par with the extractions of syntactic objects in a man I know and an easy man to please. Transfer rules which introduce raising predicates will then have to bring in the new subject explicitly. For instance, the rule for happen and digwydd, (iv) in SS4.1, will be re-written as</Paragraph>
      <Paragraph position="19"> The second point is that the shift predicate must be defined in such a way that it can cope with recursive changes in the dependency structure brought about by the occurrence of several interdependent head-switching translations. It seems that shift can be made to do this simply by having e.g. shift(H,E) affect all instances of subj(H,X) or P(X,H) (including shift(X,H))  not transferred by the transfer rule which introduced the shift(tiT, E).</Paragraph>
      <Paragraph position="20"> Together, these two stipulations enable the transfer of examples involving head-switching, argument-switching,  raising, and most types of decomposition.</Paragraph>
      <Paragraph position="21"> 5. 'IYansfer of Coordinate Structures 5.1. Problems in transfer of coordinate structures  Though transfer of coordinated structures often interacts with transfer of other translation units to produce miserable results in translation, few attempts have been reported to formalize this problem. We here divide the problem into two categories:  As is understood from its translation, &amp;quot;wear&amp;quot; is translated &amp;quot;kaburu&amp;quot; or &amp;quot;haku&amp;quot; in Japanese, depending on whether its object is something worn on the head or on the foot(or leg). This means that, in this example, coordination of objects in English should be altered to that of verb phrases in Japanese.</Paragraph>
      <Paragraph position="22"> This type of knowledge for lexical choice is very common in a transfer or bi-lingual dictionary, and plays an essential role in lexical transfer of most current transfer-based MT systems. The problem arises because neither a transfer program or a transfer-rule writer expects such an awkward problem ot be caused by coordination. To translate &amp;quot;wear&amp;quot; into &amp;quot;kaburu&amp;quot; in the above example, a rule writer may usually write the following rule in our notation1: 1 In this section, we consider translation whose source and target logical forms are on the left and right sides of a transfer rule. For the sake of simplicity, transfer rules hereafter are described as uni-directional ones.</Paragraph>
      <Paragraph position="23"> wear(X) , kaburu(X)iff obj(X,Y)&amp;HAT(Y) But the condition part of this rule is implicitly expected to be interpreted as follows.</Paragraph>
      <Paragraph position="24"> wear(X) , kaburu(X) iff V Y obj(X,Y)&amp;HAT(Y) The precise definition may change depending on how the coordinate structure is represented. But the point is that &amp;quot;wear&amp;quot; may be translated &amp;quot;kaburu&amp;quot; only if all the objects have a feature of being a &amp;quot;HAT&amp;quot;.</Paragraph>
      <Paragraph position="25"> A simple transfer algorithm, for example, may choose the target word when it finds the first applicable transfer rule for a source word: this algorithm may produce &amp;quot;boushito kutsu-wo kaburu&amp;quot; for the sentence (la), which means that the semantic relation between &amp;quot;wear&amp;quot; and &amp;quot;shoes&amp;quot; is ignored.</Paragraph>
      <Paragraph position="26"> The second type of problems occurs when one of the coordinated objects triggers a complex transfer which is described in SS3. This type of problem can be seen in the following translation.</Paragraph>
      <Paragraph position="27"> He committed murder then suicide.</Paragraph>
      <Paragraph position="28"> kare-ga satsujin-wo okashi, jisatsu-shita.</Paragraph>
      <Paragraph position="29"> he-subj murder-obj commit commlt-ed suicide This problem is more complicated than the previous one because complex transfer, in this example &amp;quot;many-to-one transfer&amp;quot;, causes a structural change.</Paragraph>
      <Paragraph position="30"> There are several possible solutions to this problem. The simplest one is that a transfer-rule writer writes all the transfer rules which explicitly describe every possible sub-structure with coordination. This is of course unrealistic. Another solution is to make a transfer program which modifies the transfer result dynamically when, for example, a source word is given two different target words. But such a dynamic modification of the result during transfer is against our policy of logic-based transfer, because this means gradual transformation of the source structure and therefore transfer cannot be formalized as logical inference.</Paragraph>
    </Section>
    <Section position="2" start_page="257" end_page="259" type="sub_section">
      <SectionTitle>
5.2. Transfer with Coordinate
Expansion
Coordinate Expansion Hereafter we concentrate on
</SectionTitle>
      <Paragraph position="0"> a case where coordinated objects of a verb cause the problem, though there is apparently an example where other cases such as &amp;quot;subj&amp;quot; cause the same problem. The  basic requirement in logic-based transfer is that coordination of objects should be reorganized to that of verb phrases or sentences, which is not supposed to cause problems in transfer. We call this reorganization &amp;quot;coordinate expansion&amp;quot;. The following is a part of logical form for (la), which involves a coordinate structure.</Paragraph>
      <Paragraph position="1"> wear(w)&amp;obj (w,o)&amp;coord(o,ol)&amp;hat (ol)&amp; coord(o,o2)&amp;shoe(o2) In this form, o is a linguistic object, and a predicate coord represents a relation between the linguistic object and its constituents. The following is a result of expansion. null</Paragraph>
      <Paragraph position="3"> The most naive and simplest strategy using this expansion is to expand every coordination within a sentence and represent it in sentence coordination before transfer. This transfer result will be reorganized again into an appropriate representation of coordination in the target language. But this solution seems inefficient from the computational point of view because caseswhere expansion is necessary are rare . Unnecessary expansion and reorganization of coordinate structures should be avoided. The strategy we propose executes coordination expansion only if it is necessary\[5\] (See Fig. 1).</Paragraph>
      <Paragraph position="4"> Transfer is divided into three phases; in the first phase, logical forms are expanded if expand rules (explained below) find the necessity of coordinate expansion. This process continues as long as the necessity remains. In the second phase, transfer described in previous sections is executed. Finally, in the third phase, coordination is reorganized if the target language has a more appropriate structure for coordination than the second phase result.</Paragraph>
      <Paragraph position="5"> (Consider the translation of (lb). Without reorganizing coordination, the transfer result will contain two &amp;quot;wear&amp;quot;s .) The following is an expand rule which detects the necessity of expansion concerned with translation of &amp;quot;wearing a hat&amp;quot;.</Paragraph>
      <Paragraph position="7"> In the rule, ezpand(Y,X) means that coordination of the level Y should be expanded to that of X. This rule suggests that coordinate expansion is necessary if the object of &amp;quot;wear&amp;quot; is a coordinate structure, and one constituent is a HAT while another isn't.</Paragraph>
      <Paragraph position="8"> We assume that expand rules are produced automatically from transfer rules before the actual transfer operation. The simplest way of extracting such a rule refers only one transfer rule at one time. This means that the necessity of expansion can be detected not by creating expand rules but by referring transfer rules in actual transfer phase. But the former approach seems essential if we wish to optimize expansion detection.</Paragraph>
      <Paragraph position="9"> Other examples Expand rules from a transfer rule which involves a structural change are a little different in forms to the previous case. The following are a transfer rule for translating &amp;quot;commit suicide&amp;quot; into Japanese and its expand rule.</Paragraph>
      <Paragraph position="10">  This case is more complicated than the previous ones because the transfer rule refers to two semantic features. Therefore we will get two expand rules, one of which is the following rule.</Paragraph>
      <Paragraph position="12"> In addition, we need another expand rule which checks another type of coordinate construction. This rule will apply when the system translates &amp;quot;she has long hair and beautiful eyes&amp;quot;.</Paragraph>
      <Paragraph position="14"> Discussion Our coordinate expansion detection works even if a coordinate structure has more than two constituents. What we have to consider is an appropriate expansion algorithm. For example, in translating (4a), an appropriate expansion should keep coordination of &amp;quot;shoes&amp;quot; and &amp;quot;stockings&amp;quot;, as shown in (45), because both satisfy a semantic constraint on which the system chooses &amp;quot;haku&amp;quot; as the translation of &amp;quot;wear&amp;quot;. Otherwise reorganizing a coordination in the generation phase is inevitable.</Paragraph>
      <Paragraph position="15"> (4a) She wears a hat, stockings and shoes.</Paragraph>
      <Paragraph position="16"> (4b) She wears a hat and wears stockings and shoes.</Paragraph>
      <Paragraph position="17"> Reorganization of a coordination in the target language does not only occur as outlined in the above case. Since the coordinate expansion is completely separate from the actual transfer process, transfer rules which do not cause problems might be used.</Paragraph>
      <Paragraph position="18"> There is still an open problem to be solved with regard to the transfer of coordination with &amp;quot;expansion&amp;quot;; expansion is not always straightforward. There is often a case where coordinate expansion is impossible without deep understanding of the sentences, or it is impossible in that it may change their semantic structures. For example, the sentence (55) cannot be the expansion of (ha) though it seems so at first glance.</Paragraph>
      <Paragraph position="19"> (ha) I saw a group of men and women.</Paragraph>
      <Paragraph position="20"> (55) I saw a group of men and I saw a group of women.</Paragraph>
    </Section>
  </Section>
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