Thanks go to Stefan Gastinger, Michael Gengenbach, Rolf Hennicker, Robert Stabl (reusability group of Spectrum) and to Bernhard Reus, Thomas Streicher (semantic group of Spectrum) for inspiring discussions and many ideas which are reflected in this paper. Thanks go also to the working group on formal methods of the First Int. Workshop on Software Reuseability (Dortmund 1991) and in particular to Rene Jaquart and Larry Latour for many new insights into formal support for software reuse.