Many of the ideas presented here were first explored in my PhD thesis ; I was privileged to carry out that work under the supervision of the late John Kelly. A large number of people have since helped and challenged me in trying to refine the analysis and arguments. I am particularly indebted to Chris Langton, Glen Ropella, Noel Murphy, Tim Taylor, Mark Bedau and Moshe Sipper; the latter also provided critical encouragement to finally write this particular paper. I am grateful also to the reviewers for comprehensive and constructive criticism. Financial support for the work has been provided by the Research Institute in Networks and Communications Engineering (RINCE) at Dublin City University.
Copyright © 2000 All Rights Reserved.