Gregor Kiczales is a Member of the Research Staff in the System Sciences Laboratory at Xerox Palo Alto Research Center. Jim des Rivi res is a Member of the Research Staff in the System Sciences Laboratory at Xerox Palo Alto Research Center. Daniel G. Bobrow is a Research Fellow in the Intelligent Systems Laboratory, Xerox Palo Alto Research Center, editor-in-chief of the Journal of Artificial Intelligence, and Chair of the Governing Board of the Cognitive Science Society.