XI: A Simple Prolog-based Language for Cross-Classification and Inheritance

This paper describes a simple Prolog-based knowledge representation language called XI – X for cross-classification and I for inheritance – which is designed to represent knowledge about individuals, about classes of individuals, and about inclusion relations between classes of individuals. XI allows for straightforward definitions of cross-classification hierarchies and for the association of arbitrary attribute-value information with classes or with individuals. XI also provides a simple inheritance mechanism which allows attribute values to be inherited by classes or individuals lower in the hierarchy. XI is simple, flexible, theoretically well-founded, and fully implemented. The utility of the language is demonstrated by discussing how it has been motivated by, and used in, a natural language processing system to represent the conceptual and world knowledge needed to perform co-reference resolution.