Parsing D-tree Grammars

In this paper we will describe a polynomial time Earley-style predictive parser for D-Tree Grammars (DTG). DTG were designed to share some of the advantages of TAG while overcoming some of its limitations. In developing parsers for TAG it has turned out to be useful to make use of a direct mapping from TAG to Linear Indexed Grammars (LIG) and to develop algorithms that work with the resulting LIG. In the case of DTG we use a similarly direct mapping into what we call Linear Prioritized Multiset Grammar (LPMG). This makes it possible to give a straightforward statement of the parsing algorithm. (1995), we deene a new grammar formalism, called D-Tree Grammars (DTG), which arises from work on Tree-Adjoining Grammars (TAG) (Joshi et al., 1975). A salient feature of TAG is the extended domain of locality it provides. Each elementary structure can be associated with a lexical item (as in Lexicalized TAG (LTAG) (Joshi and Schabes, 1991)). Properties related to the lexical item (such as subcategorization, agreement, certain types of word order variation) can be expressed within the elementary structure (Kroch, 1987; Frank, 1992). In addition, TAG remain tractable, yet their generative capacity is suucient to account for certain syntactic phenomena that, it has been argued, lie beyond Context-Free Grammars (CFG) (Shieber, 1985). TAG, however, has two limitations. The rst problem is that the TAG operations of substitution and adjunction do not map cleanly onto the relations of complementation and modiication. A second problem has to do with the inability of TAG to provide analyses for certain syntactic phenomena. We will discuss the rst issue in some detail here, and refer to Rambow et al. (1995) for a broader discussion of linguistic data that motivates the deenition of DTG. In LTAG, the operations of substitution and adjunction relate two lexical items. It is therefore natural to interpret these operations as establishing a direct linguistic relation between the two lexical items, namely a relation of complementation (predicate-argument relation) or of modiica-tion. In purely CFG-based approaches, these relations are only implicit. However, they represent important linguistic intuition, they provide a uniform interface to semantics, and they are, as Sch-abes and Shieber (1994) argue, important in order to support statistical parameters in stochastic frameworks and appropriate adjunction constraints in TAG. In many frameworks, complementation and modiication are in fact made explicit: LFG (Bresnan and Kaplan, 1982) provides a separate functional (f-) structure, and …