Aarhus Univ., Comp. Science Dep., DAIMI PB-152, September 1982.
Also in: Informatik-Fachberichte 66: Application and Theory of Petri Nets, pages 132-148. Selected Papers from the Third European Workshop on Application and Theory of Petri Nets, Varenna, Italy, September 27--30, 1982 / Pagnoni, A.; Rozenberg, G. (eds.) --- Springer-Verlag, 1983.