Using Bidirectional Semantic Rules for Generation

This paper describes the use of a system of semantic rules to generate noun compounds, vague or polysemous words, and cases of metonymy. The rules are bidirectional and are used by the understanding system to interpret the same constructions. I n t r o d u c t i o n In generation systems that are paired with understanding systems, bidirectionality is desirable for reasons that are both theoretical (a single model of linguistic behaviour) and practical (shorter development time, greater consistency, etc.) 1. Recently, [Shieber et al. 89] and [Calder at al. 89] have presented generation algor i thms tha t share both semantics and syntax with the understanding system. This paper presents an extension of these algori thms to deal with phenomena that have often been lumped together under 'pragmat ics ' , namely noun compounding, metonymy (the use of a word to refer to a related concept), and vague or polysemous words like "have." The difficulty with these constructions is tha t they are productive, and cannot be handled easily by simply listing meanings in a lexicon. Taking noun compounding as an example, we have "corn oil" and "olive oil" referring to oil made from corn or olives. We could add a lexical sense for "corn" meaning "made from corn," but then we face an explosion in the size of the lexicon, and an inability to understand or generate novel compounds: if we acquire "safflower" as the name of a plant, we would like the system to be able to handle "safflower oil" immediately, but this won' t be possible if we need a separate lexical sense to handle compounding. The system will be more robust (and the lexicon more compact) if we can derive the desired sense of "safflower" from the basic noun sense when we need it. We have therefore developed a system of bidirectional semantic rules to handle these phenomena at the appropriate level of generality. IF or more detailed arguments along these lines, see [Appelt 87], [Shieber 88], [Jacobs 88a]. We have implemented these rules in Common Lisp as par t of the KBNL system [Barnett et M. 90] at MCC, but nothing depends on the idiosyncracies of our formalisms or implementa t ion , so the technique is compatible with a wide variety of theories of the kinds of relations that are likely to occur in these constructions, as in, e.g., [Finin 80] for noun compounds and [Nunberg 78] for oblique reference. T h e F r a m e w o r k The algorithms for recognition and generation use an agenda-based blackboard for communication and control [Cohen et al. 89]. Our syntax component uses an extension of Categorial Unification Grammar [Wittenburg 86] as the phrase-structure component of an LFG-style functional representation (f-struCture), and the semantic component maps from this representat ion to sets of assertions in the interface language of the CYC knowledge base [Lenat et al. 90]. Semantic rules map partial semantic interpretations onto other partial interpretations. They consist of a left-hand side and a right-hand side, each consisting of one or more templates, plus a mechanism for mapping an instantiation of either set of templates onto an instantiat ion of the other set. The intuitive semantics of these rules is that any interpretation that matches the left-hand side licenses a second interpretation matching the right-hand side. For example, we can use the name of an author to refer to his works ("I read Shakespeare"), and the corresponding semantic rule states that the existence of an NP denoting an artist licences the use of the same NP to refer to his works. The generation system applies the rules in a backward-chaining direction, while the understanding system runs them forward. A later section contains a fuller discussion of the implementat ion of the rules, while the next sections discuss their use at runtime.