Preserving Semantic Dependencies in Synchronous Tree Adjoining Grammar

Preserving Semantic Dependencies in Synchronous Tree Adjoining Grammar* William Schuler University of Pennsylvania 200 South 33rd Street Philadelphia, PA 19104 USA schuler@linc, cis. upenn, edu Abstract Rambow, Wier and Vijay-Shanker (Rainbow et al., 1995) point out the differences between TAG derivation structures and semantic or predicate- argument dependencies, and Joshi and Vijay- Shanker (Joshi and Vijay-Shanker, 1999) de- scribe a monotonic compositional semantics based on attachment order that represents the desired dependencies of a derivation without un- derspecifying predicate-argument relationships at any stage. In this paper, we apply the Joshi and Vijay-Shanker conception of compositional semantics to the problem of preserving seman- tic dependencies in Synchronous TAG transla- tion (Shieber and Schabes, 1990; Abeill~ et al., 1990). In particular, we describe an algorithm to obtain the semantic dependencies on a TAG parse forest and construct a target derivation forest with isomorphic or locally non-isomorphic dependencies in O(n 7) time. 1 Introduction The primary goal of this paper is to solve the problem of preserving semantic dependencies in Isomorphic Synchronous Tree Adjoining Gram- mar (ISTAG) (Shieber, 1994; Shieber and Sch- abes, 1990), a variant of Tree Adjoining Gram- mar (Joshi, 1985) in which source and target elementary trees are assembled into isomorphic derivations. The problem, first described in Rambow, Wier and Vijay-Shanker (Rainbow et al., 1995), stems from the fact that the TAG derivation structure - even using a flat adjunc- tion of modifiers (Schabes and Shieber, 1994) - deviates from the appropriate dependency *The author would like to thank Karin Kipper, Aravind Joshi, Martha Palmer, Norm Badler, and the anonymous reviewers for their valuable comments. This work was partially supported by NSF Grant SBP~8920230 and ARO Grant DAAH0404-94-GE-0426. structure in certain cases. This can result in translation errors. For example, if we parse sentence (1), (1) X is supposed to be able to fly. using the trees in Figure 1, we get the following derivation:l a:fly I 131 :be-able-to(VP) I j32:is-supposed-to(VP) with the auxiliary is-supposed-to adjoining at the VP to predicate over be-able-to and the aux- iliary be-able-to adjoining at the VP to predi- cate over fly. If we then try to assemble an iso- morphic tree in a language such as Portuguese (which makes less use of raising verbs) using the ISTAG transfer rules in Figure 2, we will be forced into an ill-formed derivation: : voar I ;31 :~-capaz-de (VP) I /~2 :~-pressuposto-que (S ?) because the raising construction is-supposed- to translates to a bridge construction d- pressuposto-que and cannot adjoin anywhere in the tree for ~-capaz-de (the translation of be- able-to) because there is no S-labeled adjunction site. The correct target derivation: a:voar ~l:~-capaz-de(VP) ~2:~-pressuposto-que(S) 1The subject is omitted to simplify the diagram. 88 VP VP Vo VP Vo VP is Vo VP [ [ ~ able Vo VP* supposed Vo VP* [ [ to to S NP$ VP I Vo I fly Figure 1: Sample elementary trees for "supposed to be able to fly" which yields the translation in sentence (2), (2) t~ pressuposto que X …

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