Transforming Syntactic Graphs Into Semantic Graphs

TRANSFORMING SYNTACTIC GRAPHS INTO SEMANTIC GRAPHS* Hae-Chang Rim Jungyun Seo Robert F. Simmons Department of Computer Sciences and Artificial Intelligence Laboratory Taylor Hall 2.124, University of Texas at Austin, Austin, Texas 78712 ABSTRACT In this paper, we present a computational method for transforming a syntactic graph, which represents all syntactic interpretations of a sentence, into a semantic graph which filters out certain interpretations, but also incorporates any remaining ambiguities. We argue that the result- ing ambiguous graph, supported by an exclusion matrix, is a useful data structure for question an- swering and other semantic processing. Our re- search is based on the principle that ambiguity is an inherent aspect of natural language communi- cation. INTRODUCTION In computing meaning representations from natural language, ambiguities arise at each level. Some word sense ambiguities are resolved by syn- tax while others depend on the context of dis- course. Sometimes, syntactic ambiguities are re- solved during semantic processing, but often re- main even through coherence analysis at the dis- course level. Finally, after syntactic, semantic, and discourse processing, the resulting meaning structure may still have multiple interpretations. For example, a news item from Associated Press, November 22, 1989, quoted a rescued hostage, "The foreigners were taken to the Estado Mayor, army headquarters. I left that hotel about quarter to one, and by the *This work is sponsored by the Army Research Office under contract DAAG29-84-K-0060. 47 time I got here in my room at quarter to 4 and turned on CNN, I saw myself on TV getting into the little tank," Blood said. The article was datelined, Albuquerque N.M. A first reading suggested that Mr. Blood had been flown to Albuquerque, but further thought sug- gested that "here in my room" probably referred to some sleeping section in the army headquarters. But despite the guess, ambiguity remains. In a previous paper [Seo and Simmons 1989] we argued that a syntactic graph -- the union of all parse trees -- was a superior representation for further semantic processing. It is a concise list of syntactically labeled triples, supported by an ex- clusion matrix to show what pairs of triples are incompatible. It is an easily accessible represen- tation that provides succeeding semantic and dis- course processes with complete information from the syntactic analysis. Here, we present methods for transforming the syntactic graph to a func- tional graph (one using syntactic functions, SUB- JECT, OBJECT, IOBJECT etc.) and for trans- forming the functional graph to a semantic graph of case relations. BACKGROUND Most existing semantic processors for natural language systems (NLS) have depended on a strat- egy of selecting a single parse tree from a syntac- tic analysis component (actual or imagined). If semantic testing failed on that parse, the system would sel~,ct another -- backing up if using a top- down parser, or selecting another interpretation vpp(8) ppn 0 1 (SNP saw John) 1 2 (VNP saw man) 2 3 (DET man a) 3 4 (NPP man on) 4 5 (VPP saw on) 5 6 …

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