Upper Modeling: organizing knowledge for natural language processing

A general, reusable computational resource has been developed within the Penman text generation project for organizing domain knowledge appropriately for linguistic realization. This resource, called the u p p e r model , provides a domainand task-independent classification system' that supports sophisticated natural language processing while significantly simplifying the interface between domain-specific knowledge and general linguistic resources. This paper presents the results of our experiences in designing and using the upper model in a variety of applications over the past 5 years. In particular, we present our conclusions concerning the appropriate organization of an upper model, its domainindependence, and the types of interrelationships that need to be supported between upper model and grammar and semantics. I n t r o d u c t i o n : i n t e r f a c i n g w i t h a t e x t g e n e r a t i o n s y s t e m Consider the task of interfacing a domain-independent, reusable, general text generation system with a particular application domain, in order to allow that application to express system-internal information in one or more natural languages. Internal information needs to be related to strategies for expressing it. This could be done in a domain-specific way by coding how the application domain requires its information to appear. This is clearly problematic, however: it requires detailed knowledge on the part of the system builder both of how the generator controls its output forms and the kinds of information that the application domain contains. A more general solution to the interfacing problem is thus desirable. We have found that the definition of a mapping between knowledge and its linguistic expression is facilitated if it is possible to classify any particular instances of facts, states of affairs, situations, etc. that occur in terms of a set of general objects and relations of specified types that behave systematically with respect to their possible linguistic realizations. This approach has been followed within the PENMAN text generation system [Mann and Matthiessen, 1985; The Penman Project, 1989] where, over the past 5 years, we have been developing and using an extensive, domainand task-independent organization of knowledge that supports natural language generation: this level of organization is called the u p p e r mode l [Bateman et aL, 1990; Mann, 1985; Moore and Arens, 1985]. The majority of natural language processing systems currently planned or under development are now recognizing the necessity of some level of abstract 'semantic' organization similar to the upper model that classifies knowledge so that it may be more readily expressed linguisticaUy. 1 However, they mostly suffer from either a lack of theoretical constraint concerning their internal contents and organization and the necessary mappings between them and surface realization, or a lack of abstraction which binds them too closely with linguistic form. It is important both that the contents of such a level of abstraction be motivated on good theoretical grounds and that the mapping between that level and linguistic form is specifiable. Our extensive experiences with the implementation and use of a level of semantic organization of this kind within the PENMAN system now permit us to state some clear design criteria and a well-developed set of necessary functionalities. T h e U p p e r M o d e l ' s C o n t r i b u t i o n t o t h e S o l u t i o n t o t h e I n t e r f a c e P r o b l e m : D o m a i n i n d e p e n d e n c e a n d r e u s a b i l i t y The upper model decomposes the mapping problem by establishing a level of linguistically motivated knowledge organization specifically constructed as a reponse XIncluding, for example: the Functional Sentence Structure o f XTRA: [Allgayer et al., 1989]; [Chen and Cha, 1988]; [Dahlgren et al., 1989]; POLYGLOSS: [Emele et ai., 1990]; certain of the Domain and Text Structure Objects of SPOKESMAN: [Meteer, 1989]; TRANSLATOR: [Nixenberg et aL, 1987]; the Semantic Relations of ~UROTa^-D: [Steiner et al., 1987]; JANUS: [Weischedel, 1989]. Space naturally precludes detailed comparisons here: see [Bateman, 1990] for further discussion.