Negations and exceptions in Topoi

Topoi is a language for constructing arguments. Rules are labelled with a context in which they apply, so that different, or even conflicting, rules can be supplied to represent different points of view. This in turn allows the construction through meta-level programming of differing or conflicting arguments. Although first applied in a legal application, the language can be used in more general situations. The paper first presents a brief overview of the main features of the language. It subsequently introduces the three types of negation-explicit negation, negation by failure, and closed world negation-provided by the language. Finally it describes a useful mechanism for writing rules that allow exceptions