NKRL, a Knowledge Representation Language for Narrative Natural Language Processing

NKRL is a conceptual language which intends to provide a normalised, pragmatic description of the semantic contents (in short, the "meaning") of NL narrative documents. We introduce firstly the general architecture of NKRL, and we give some examples of its characteristic features. We supply, afterward, some sketchy information about the inference techniques and the NLP procedures associated with this language. 1 . I n t r o d u c t i o n N K R L (Narrat ive Knowledge Representa t ion Language) aims to propose some possible, pragmatic solutions for the set up of a standardised description of the semantic contents (in short, the "meaning") of natural language (NL) n,'uTative documents. With the term "nan'ative documents" we denote here NL texts of an industrial and economic interest con'esponding, e.g., to news stories, corporate documents, normative texts, intelligence messages, etc. The NKRL code can be used according to two main modalities. It can be employed as a standard vehicle for the interchange of content information about narrative documents. It can also bc utilised to support a wide range of industrial applications, like populating large knowledge bases which can support, thereafter, all sort of "intelligent" applications (advanced expert systems, case-based reasoning, intelligent information retrieval, etc.). NKRL is a fully implemented language ; the most recent versions have been rcalised in the frmnework of two European projects : NOMOS, Esprit P5330, and COBALT, LRE P61011. 2 . The arch i tec ture o f N K R L NKRL is a two layer language. The lower layer consists of a set of general tools which are structured into several integrated components, four in our case. The descriptive component concerns the tools used to produce the formal representations (called predicative templates) of general classes of narrative events, like "moving a generic object", "formulate a need", "be present somewhere". Predicative templates are characterised by a threefold format, where the central piece is a semantic predicate (a primitive, like BEHAVE, EXPERIENCE, MOVE, PRODUCE etc.) whose arguments (role fillers) are introduced by roles as SUBJ(ect), OBJ(ect), SOURCE, DEST(ination), etc. ; the data structures proper to the descriptive component are then similar to the case-grammar structures. Templates are structured into a hierarchy, H TEMP(lates), corresponding, therefore, to a "taxonomy of events". Templates ' instances (predicative occurrences), i.e., the NKRL representation of single, specific events like "Tomorrow, I will move the wardrobe", "Lucy was looking for a taxi", "Peter lives in Paris", ,are in the domain of the factual component. The definitional component supplies the NKRL representations, called concepts, of all the general notions, like physical_entity, human_being, taxi_, city_, etc., which can play the role of arguments within the data su'uctures of the two components above. The concepts correspond to sets or collections, organised according to a generalisation/specialisation (tangled) hier~chy which, for historical reasons, is called H_CLASS(es). The data structures used for the concepts are, substantially, frame-like structures ; H_CLASS corresponds relatively well, therefore, to the usual ontologies of terms. The enumerative component of NKRL concerns the formal representation of the instances (concrete, countable ex,'unples, see lucy_, wardrobe_l, taxi_53) of the concepts of H CLASS ; their formal representations take the name of individuals. Throughout this paper, we will use the italic type style to represent a "concept", the roman style to represent an "individual_". The upper layer of NKRL consists of two parts. The first is a "catalogue", giving a complete description of the formal characteristics and the modalities of use of the well-formed, "basic templates" (like "moving a generic object" mentioned above) associated with the language presently, about 150, pertaining mainly to a (very general) socio-economicopolitical context where the m~fin characters are human being~ or social bodies. By means of proper specialisation operations it is then possible to obtain, from the basic templates, the (specific) "derived" templates that could be concretely needed to implement a particular, practical application e.g., "move an industrial process" and the corresponding occurrences. In NKRL, the set of legal, basic templates can be considered, at least in a first approach, ,as fixed. Analogously, the general concepts which pertain to the upper levels of H_CLASS such as human_being, physicalentity, modality_, etc. form a sort of upper-level, ilwariable ontology. 3. Some characteristic NKRL features Fig. 1 supplies a simple exmnple of NKRL code. It translates a small fragment of COBALT news : "Milan, October 15, 1993. qhe financial daily 11 Sole