INTRODUCTION The hypergraphical architecture of the Web has been used to support claims that the Web will make Internetbased services really user-friendly. However, at its current state, the Web has not achieved its goal of providing easy access to online information. Being an almost unstructured and heterogeneous environment it creates an information overload and places obstacles in the way us...

The braid group Bn is the mapping class group of an n-punctured disk. An n-braid is reducible if it has an invariant essential curve system in the punctured disk. We show that for some class of reducible braids (including split braids), finding a reduction system is as easy as finding one element in the ultra summit set.

At any stage in a well-run conversation, a certain amount is presupposed. The parties to the conversation take it for granted; or at least they purport to, whether sincerely or just “for the sake of the argument”. Presuppositions can be created or destroyed in the course of a conversation. This change is rule-governed, at least up to a point. The presuppositions at time r’ depend, in a way abou...