An Approach to Multilevel Semantics for Applied Systems

AN APPROACH TO MULTILEVEL SEMANTICS FOR APPLIED SYSTEMS Alberto Lavelli Bernardo Magnini Carlo Strapparava IRST, Istituto per la Ricerca Scientifica e Tecnologica I - 38050 Povo TN, Italy e-mail: magnini@irst.it Abstract Multilevel semantics has been proposed as a powerful architecture for semantic analysis. We propose a methodology that, while maintaining the generality of the multilevel approach, is able to establish formal constraints over the possible ways to organize the level hierarchy. More precisely, we propose a "strong" version of the multilevel approach in which a level can be defined if and only if it is possible to characterize a "meaningfulness" notion peculiar to that level. Within such an architecture each level reached during the analysis computes its meaningfulness value; this result is then handled according to modalities that are peculiar to that level. The component described in this paper was designed to be portable with respect to the application domain and so far has been tested as the semantic analysis component of two multimedial dialog systems, ALFresco and MAIA. 1. Introduction Multilevel semantics has been proposed [Scha, 1983] as a powerful architecture for semantic analysis. In this approach, interpreting a natural language sentence is a multi-stage process, which starts out with a high-level meaning representation that reflects the semantic structure of the sentence rather directly. Then translation rules, which specify how the language-oriented semantic primitives relate to those used at deeper levels of analysis, are applied. One of the advantages of the multilevel approach is that it allows a natural decomposition of complex tasks and the functional modularization of semantic analysis. However, when multilevel architecture is used in concrete applications, a simple functional approach does not solve the problem of a clear definition of the semantics for each level. This fact is evident for applied systems whose semantic component must deal with many linguistic phenomena (e.g. lexical and structural ambiguities, quantifier scoping, anaphorical references, discourse topic and focus, referent retrieval, etc.). In such systems the definition of the semantics for a level has at least two advantages: (i) modules for specific phenomena could be easily introduced within the appropriate level, provided that the module functions contribute to the definition of the semantics for that level; (ii) a better understanding of the semantic analysis would be allowed: particularly, when a sentence is rejected at a certain level, it would mean that the semantic constraints for that level have been violated. In this paper we suggest a methodology that, while maintaining the generality of the multilevel approach, is able to establish formal constraints over the possible ways to organize the level hierarchy. More precisely, we propose a "strong" version of the multilevel approach in which a level can be defined if and only if it is possible to characterize a "meaningfulness" notion peculiar to that level. Within such an architecture each level reached during the analysis computes its meaningfulness value; this result is then handled according to modalities that are peculiar to that level. We shall show how our approach to multilevel …

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