Flexible Types for a Concurrent Model

                          Franz Puntigam
    Institut für Computersprachen, Technische Universität Wien
           Argentinierstraße 8, A-1040 Vienna, Austria
             E-mail: franz@complang.tuwien.ac.at

Subtyping is undoubtedly useful for the support of incremental
refinement and reuse of software components, a crucial feature of
object-oriented languages. Types and subtyping for concurrent
languages are not yet explored sufficiently and tend to be less
flexible than desired. We propose a principle of dealing with very
flexible types in concurrent languages. The corresponding type
model ensures that ``message not understood'' errors cannot occur
even if object behavior can be dynamically modified in some ways.