Wrapping of Trees

Wrapping of Trees James Rogers Department of Computer Science Earlham College Richmond, IN 47374, USA jrogers@cs.earlham.edu Abstract We explore the descriptive power, in terms of syn- tactic phenomena, of a formalism that extends Tree- Adjoining Grammar (TAG) by adding a fourth level of hierarchical decomposition to the three levels TAG already employs. While extending the descrip- tive power minimally, the additional level of decom- position allows us to obtain a uniform account of a range of phenomena that has heretofore been dif- ficult to encompass, an account that employs uni- tary elementary structures and eschews synchro- nized derivation operations, and which is, in many respects, closer to the spirit of the intuitions under- lying TAG-based linguistic theory than previously considered extensions to TAG. 1 Introduction Tree-Adjoining Grammar (TAG) (Joshi and Sch- abes, 1997; Joshi et al., 1975) is a grammar formal- ism which comes with a well-developed theory of natural language syntax (Frank, 2002; Frank, 1992; Kroch and Joshi, 1985). There are, however, a num- ber of constructions, many in the core of language, which present difficulties for the linguistic under- pinnings of TAG systems, although not necessarily for the implemented systems themselves. Most of these involve the combining of trees in ways that are more complicated than the simple embedding pro- vided by the tree-adjunction operation. The most widely studied way of addressing these constructions within TAG-based linguistic theory (Kroch and Joshi, 1987; Kroch, 1989; Frank, 2002) has been to assume some sort of multi-component adjoining (MCTAG (Weir, 1988)), in which elemen- tary structures are factored into sets of trees that are adjoined simultaneously at multiple points. De- pending on the restrictions placed on where this ad- joining can occur the effect of such extensions range from no increase in complexity of either the licensed tree sets or the computational complexity of pars- ing, to substantial increases in both. In this paper we explore these issues within the framework of an extension of TAG that is conservative in the sense that it preserves the unitary nature of the elemen- tary structures and of the adjunction operation and extends the descriptive power minimally. While the paper is organized around particular syntactic phenomena, it is not a study of syntax itself. We make no attempt to provide a compre- hensive theory of syntax. In fact, we attempt to simply instantiate the foundations of existing the- ory (Frank, 2002) in as faithful a way as possible. Our primary focus is the interplay between the lin- guistic theory and the formal language theory. All of the phenomena we consider can be (and in prac- tice are (Group, 1998)) handled ad hoc with feature- structure based TAG (FTAG, (Vijay-Shanker and Joshi, 1991)). From a practical perspective, the role of the underlying linguistic theory is, at least in part, to insure consistent and comprehensive im- plementation of ad hoc mechanisms. From a theo- retical perspective, the role of the formal language framework is, at least in part, to insure coherent and computationally well-grounded theories. Our …

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