A Hybrid Approach to Representation in the Janus Natural Language Processor

A HYBRID APPROACH TO REPRESENTATION IN THE JANUS NATURAL LANGUAGE PROCESSOR Ralph M. Weischedel BBN Systems and Technologies Corporation 10 Moulton St. CambHdge, MA 02138 Abstract In BBN's natural language understanding and generation system (Janus), we have used a hybrid approach to representation, employing an intensional logic for the representation of the semantics of ut- terances and a taxonomic language with formal semantics for specification of descriptive constants and axioms relating them. Remarkably, 99.9% of 7,000 vocabulary items in our natural language ap- plications could be adequately axiomatlzed in the taxonomic language. 1. Introduction Hybrid representation systems have been ex- plored before [9, 24, 31], but until now only one has been used in an extensive natural language process- ing system. KL-TWO [31], based on a propositional logic, was at the core of the mapping from formulae to lexical items in the Penman generation system [28]. In this paper we report some of the design decisions made in creating a hybrid of an intensional logic with a taxonomic language for use in Janus, BBN's natural language system, consisting of the IRUS-II under- standing components [5] and the Spokesman genera- tion components. To our knowledge, this is the first hybrid approach using an intensional logic, and the first time a hybrid representation system has been used for understanding. In Janus, the meaning of an utterance is represented as an expression in WML (World Model Language)[15], which is an intensional logic. However, a logic merely prescribes the framework of semantics and of ontology. The descriptive constants, that is the individual constants (functions with no arguments), the other function symbols, and the predicate symbols, are abstractions without any detailed commitment to ontology. (We will abbreviate descriptive constants throughout the remainder of this paper as constants.) Axioms stating the relationships between the con- stants are defined in NIKL [8, 22]. We wished to ex- plore whether a language with limited expressive power but fast reasoning procedures is adequate for core problems in natural language processing. The NIKL axioms constrain the set of possible models for the logic in a given domain. Though we have found clear examples that argue for more expressive power than NIKL provides, 99.9% of the examples in our expert system and data bass applications have fit well within the constraints of NIKL. Based on our experience and that of others, the axioms and limited inference algorithms can be used for classes of anaphora resolution, interpretation of highly polysemous or vague words such as have and with, finding omitted relations in novel nomina/ compounds, and selecting modifier attachment based on selection restrictions. Sections 2 and 3 describe the rationale for our choices in creating this hybrid. Section 4 illustrates how the hybrid is used in Janus. Section 5 briefly summarizes some experience with domain- independent abstractions for organizing constants of the domain. Section 6 identifies related hybrids, and Section 7 summarizes our conclusions. 2. _Commitments to Component Hepresentation Formalisms We chose well-documented representation /an- guages in order to focus on formally …

Content truncated to 500 words