The explosive growth of application areas such as electronic commerce, ent- prise resource planning and mobile computing has profoundly and irreversibly changed our views on software systems. Nowadays, software is to be based on open architectures that continuously change and evolve to accommodate new components and meet new requirements. Software must also operate on di?- ent platforms, without recompilation, and with minimal assumptions about its operating environment and its users. Furthermore, software must be robust and ] autonomous, capable of serving a naive user with a minimum of...