Syntactic and Semantic Transfer with F-Structures

Syntactic and Semantic Transfer with F-Structures* Michael Dorna*, Anette Frank t, Josef van Genabith* and Martin C. Emele* *IMS, Universit~it Stuttgart tXerox Research Centre Europe *Dublin City University Azenbergstr. 12 6, chemin de Maupertuis Computer Applications D-70174 Stuttgart F-38240 Meylan Dublin 9, Ireland (dorna, emele}@ims, uni-stuttgart, de Anette. Frank@xrce. xerox, com j osef%compapp, dcu. ie Abstract We present two approaches for syntactic and se- mantic transfer based on LFG f-structures and compare the results with existing co-description and restriction operator based approaches, fo- cusing on aspects of ambiguity preserving trans- fer, complex cases of syntactic structural mis- matches as well as on modularity and reusabil- ity. The two transfer approaches are interfaced with an existing, implemented transfer com- ponent (Verbmobi1), by translating f-structures into a term language, and by interfacing f- structure representations with an existing se- mantic based transfer approach, respectively. 1 Introduction Target and source levels of representation in transfer-based machine translation (MT) are subject to often competing demands: on the one hand, they need to abstract away from partic- ulars of language specific surface realization to ensure that transfer is as simple and straightfor- ward as possible. On the other hand, they need to encode sufficiently fine-grained information to steer transfer. Furthermore, target and source representations should be linguistically well es- tablished and motivated levels of representa- tion. Finally, from a computational perspective they need to be sensible representations for both parsing and generation. LFG f-structures are abstract, "high-level" syntactic representations which go some way towards meeting these of- ten irreconcilable requirements. " We would like to thank H. Kamp, M. Schiehlen and the anonymous reviewers for helpful comments on ear- lier versions of this article. Part of this work was funded by the German Federal Ministry of Education, Science, Research and Technology (BMBF) in the framework of the Verbmobil project under grant 01 IV 701 N3. Correspondence-based transfer on f-structures has been proposed in (Kaplan et al., 1989). A closer look at translation problems involv- ing structural mismatches between languages - in particular head switching phenomena (Sadler and Thompson, 1991) - led to the contention that transfer is facilitated at the level of seman- tic representation, where structural differences between languages are often neutralized. Struc- tural misalignment is treated in semantics con- struction involving a restriction operator (Ka- plan and Wedekind, 1993) where f-structures are related to (possibly sets of) disambiguated se- mantic representations. Given the high potential of semantic ambigui- ties, the advantage of defining transfer on se- mantic representations could well be counter- balanced by the overhead generated by multi- ple disambiguated structures as input to trans- fer. This and the observation that many seman- tic (and syntactic) ambiguities can be preserved when translating into a target language that is ambiguous in similar ways, sheds light on the issue of the properties of representations for the task of defining transfer. In principle, the problem of semantic ambi- guity in transfer can be tackled in a number of ways. Packed ambiguity representation tech- niques (Maxwell …

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