An integrated parser for TFG with explicit tree typing

One of"the main conditions for the development of successful NLP applications is the usability of the syntactic formalisms adopted and the degree to which they facilitate syntax-semantics integration. TAG+ formalisms show a real potential for NLP applications, due to their linguistic descriptive capabilities. However, both the standard formalisms and parsing strategies are often quite complex and cannot easily be used as such for the development of NLP systems. We have .thus investigated a simplified TAG+ fonnalism, which sacrifices some of TAG's descriptive and formal properties for the sake of usability. Titls is especially relevant considering the recent success of empirical approaches to NLP which tend tobe based on very simple techniques and/or discard linguistically-motivated formalisms [Basili et al " 1996] [Appelt et al., 1993]. We report the implementation of a parser for a simplified TAG+ fonnalism, Tree Furcating Grammars (TFG), which integrales semantic processing, performing both syntactic disambiguation and the construction of a semantic representation for the sentence parsed. The parser has been developed for the purpose of real-time speech understanding of sublanguages (i.e., application-dependent vocabularies of 500-1000 words with specific, sometimes quite simptified, syntactic constructs). TAG+ formalisms were initially investigated because of their potential for syntax-semantics integration (see e.g., Abeille [1994)). We will successively describe the rationale for the TFG formalism, the principles underlying the algorithm used and a first assessment of its perfonnance. Tree Furcating Grammars are a lexicalised TAG+ formalism, in which adjunction is replaced by the furcation operation that essentially adds an additional branch to the target node in the initial tree, instead of copying the auxiliary tree under it. The furcation operation was originally introduced in segrnent grammars [De Smedt & Kempen, 1990]. A detailed comparison of furcation and adjunction has been given by Abeille [ 1991]. Though some syntactic phenomena are not properly bandled by furcation, the fact that it introduces modifiers without embedding them into the tree structure is a definite advantage for syntax-semantics integration, and was the rationale for choosing it 1 • Successive furcations do not increase tree depth and complexity, producing deriv~ trees that retain some properties of dependency trees. These can support the integrated construction of a semantic structure, based on the appropriate association of semantic functions to the tree structures (see below). We have adapted our tree representations accordingly, by distinguishing between left auxiliary trees (which have a *X root node) 2 and right auxiliary trees (X* root …