In: Girault, C.; Reisig, W.: Informatik-Fachberichte 52: Application and Theory of Petri Nets: Selected Papers from the First and Second European Workshop on Application and Theory of Petri Nets, Strasbourg, Sep. 23-26, 1980, Bad Honnef, Sep. 28-30, 1981, pages 187-192. Springer-Verlag, 1982.