Generating the XTAG English Grammar Using Metarules

Generating the XTAG English grammar using metarules Carlos A. Prolo Computer and Information Science Department University of Pennsylvania Suite 400A, 3401 Walnut Street Philadelphia, PA, USA, 19104-6228 prolo@linc.cis.upenn.edu Abstract We discuss a grammar development process used to generate the trees of the wide-coverage Lexicalized Tree Adjoining Grammar (LTAG) for English of the XTAG Project. Result of the coupling of Becker’s metarules and a simple yet principled hierarchy of rule application, the approach has been successful to generate the large set of verb trees in the grammar, from a very small initial set of trees. 1 Introduction The XTAG Project (Joshi, 2001) is an ongoing project at the University of Pennsylvania since about 1988, aiming at the development of natural language resources based on Tree Adjoining Gram- mars (TAGs) (Joshi and Schabes, 1997). Perhaps the most successful experience in it has been the construction of a wide-coverage Lexicalized TAG for English (Doran et al., 2000; XTAG Research Group, 2001), based on ideas initially developed in (Krock and Joshi, 1985). As the grammar grew larger, the process of con- sistent grammar development and maintenance be- came harder (Vijay-Shanker and Schabes, 1992). An LTAG is a set of lexicalized elementary trees that can be combined, through the operations of tree adjunction and tree substitution, to derive syntactic structures for sentences. Driven by locality princi- ples, each elementary tree for a given lexical head is expected to contain its projection, and slots for its arguments (e.g., (Frank, 2001)). Keeping up with these principles, one can easily see that the number of required elementary trees is huge for a grammar with reasonable coverage of syntactic phenomena. Under the XTAG project, for engineering reasons, the grammar has been split up in (roughly) two main components1: a set tree templates lexicalized by a 1For a more accurate description of the XTAG system ar- chitecture, see (XTAG Research Group, 2001) or (Doran et al., syntactic category, and a lexicon with each word selecting its appropriate tree templates. Figure 1 shows typical grammar template trees that can be selected by lexical items and combined to generate the structure in Figure 2. The derivation tree, to the right, contains the history of the tree grafting pro- cess that generated the derived tree, to the left.2 NP N S NP V NP VP VP VP PP P NP DT NP * NP* np vt det ppright Figure 1: An example of Tree Adjoining Grammar DT NP NP* N PP P * VP V NP N NP N VP S [John] [saw] [Mary] [from] [the] [window] Derived tree Derivation tree vt[saw] np[John] np[Mary] pp[from] np[window] det[the] Figure 2: Derivation of John saw Mary from the win- dow Although various syntactic categories have mul- tiple syntactic frames available (e.g., prepositions may have different kinds of arguments, nouns and adjectives may have arguments or not, etc.), it is the verbs that exhibit the most wild variety of do- mains of locality: from the 1004 template trees in 2000). 2For a more comprehensive introduction to …

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