Unification-Based Tree Adjoining Grammars

Many current grammar formalisms used in computational linguistics take a unification-based approach that use structures (called feature structures) containing sets of feature-value pairs. In this paper, we describe a unification-based approach to Tree Adjoining Grammars (TAG). The resulting formalism (UTAG) retains the principle of factoring dependencies and recursion that is fundamental to TAGs. We also extend the definition of UTAG to include the lexicalized approach to TAGs (see [Schabes et al., 1988]). We give some linguistic examples using UTAG and informally discuss the descriptive capacity of UTAG, comparing it with other unification-based formalisms. Finally, based on the linguistic theory underlying TAGs, we propose some stipulations that can be placed on UTAG grammars. In particular, we stipulate that the feature structures associated with the nodes in an elementary tree are bounded (there is an analogous stipulation in GPSG). Grammars that satisfy these stipulations are equivalent to TAG. Thus, even with these stipulations, UTAGs have more power than CFG-based unification grammars with the same stipulations. Comments University of Pennsylvania Department of Computer and Information Science, Technical Report No. MSCIS-91-25. This technical report is available at ScholarlyCommons: http://repository.upenn.edu/cis_reports/450 Unification-Based Tree Adjoining Grammars MS-CIS-91-25 LINC LAB 199 K. Vijay-S hanker (University of Delaware) Aravind K. Joshi (University of Pennsylvania) Department of Computer and Information Science School of Engineering and Applied Science University of Pennsylvania Philadelphia, PA 19104-6389 March 1991 [[To appear in "Unification-Based Grammars" (ed. Jurgen Wedekind), MIT PRESS 199111 UnificationBased Tree Adjoining Grammars't K. Vijay-Shanker Aravind K. Joshi Dept. of Computer & Information Sciences Dept. of Computer & Information Science University of Delaware University of Pennsylvania Newark, DE 19711 Philadelphia, PA 19104 US.A U.S.A.