Syntactic and Semantic Transfer with F-Structures

We present two approaches for syntactic and semantic transfer based on LFG f-structures and compare the results with existing co-description and restriction operator based approaches, focusing on aspects of ambiguity preserving transfer, complex cases of syntactic structural mismatches as well as on modularity and reusability. The two transfer approaches are interfaced with an existing, implemented transfer component (Verbmobi1), by translating f-structures into a term language, and by interfacing fstructure representations with an existing semantic based transfer approach, respectively. 1 I n t r o d u c t i o n 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 particulars of language specific surface realization to ensure that transfer is as simple and straightforward 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 established and motivated levels of representation. 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 often irreconcilable requirements. " We would like to thank H. Kamp, M. Schiehlen and the anonymous reviewers for helpful comments on earlier 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 involving 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 semantic representation, where structural differences between languages are often neutralized. Structural misalignment is treated in semantics construction involving a restriction operator (Kaplan and Wedekind, 1993) where f-structures are related to (possibly sets of) disambiguated semantic representations. Given the high potential of semantic ambiguities, the advantage of defining transfer on semantic representations could well be counterbalanced by the overhead generated by multiple disambiguated structures as input to transfer. This and the observation that many semantic (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 ambiguity in transfer can be tackled in a number of ways. Packed ambiguity representation techniques (Maxwell III and Kaplan, 1993) could be integrated with the approach in (Kaplan and Wedekind, 1993). In the linear logic based semantics of (Dalrymple et al., 1996) scope ambiguities are accounted for in terms of alternative derivations of meaning assignments from a set of meaning constructors. Ambiguity preserving semantic transfer can be devised on sets of meaning constructors rather than disambiguated meanings (Genabith et al., 1998). Transfer on packed representations is considered