tailieunhanh - Báo cáo khoa học: "Parsing with an Extended Domain of Locality"

One of the claimed benefits of Tree Adjoining G r a m m a r s is that they have an extended domain of locality (EDOL). We consider how this can be exploited to limit the need for feature structure unification during parsing. We compare two wide-coverage lexicalized g r a m m a r s of English, LEXSYS and XTAG, finding that the two grammars exploit EDOL in different ways. | Proceedings of EACL 99 Parsing with an Extended Domain of Locality John Carroll Nicolas Nicolov Olga Shaumyan Martine Smets David Weir School of Cognitive and Computing Sciences University of Sussex Brighton BN1 9QH UK Abstract One of the claimed benefits of Tree Adjoining Grammars is that they have an extended domain of locality EDOL . We consider how this can be exploited to limit the need for feature structure unification during parsing. We compare two wide-coverage lexicalized grammars of English LEXSYS and XTAG finding that the two grammars exploit EDOL in different ways. 1 Introduction One of the most basic properties of Tree Adjoining Grammars tags is that they have an extended domain of locality edol Joshi 1994 . This refers to the fact that the elementary trees that make up the grammar are larger than the corresponding units the productions that are used in phrase-structure rule-based frameworks. The claim is that in Lexicalized TAGS ltags the elementary trees provide a domain of locality large enough to state co-occurrence relationships between a lexical item the anchor of the elementary tree and the nodes it imposes constraints on. We will call this the extended domain of locality hypothesis. For example w z-movement can be expressed locally in a tree that will be anchored by a verb of which an argument is extracted. Consequently features which are shared by the extraction site and the wft-word such as case do not need to be percolated but are directly identified in the tree. Figure 1 shows a tree in which the case feature at the extraction site and the w z-word share the same The anchor substitution and foot nodes of trees are marked with the symbols o ị and respectively. Words in parenthesis are included in trees to provide examples of strings this tree can derive. Much of the research on TAGS can be seen as illustrating how its EDOL can be exploited in various ways. However to date only indirect evidence has been given regarding the beneficial