TU München, Institut für Informatik, Bericht Nr. TUM-I8614, September 1986.
Also in: Brandenburg, F.J.; Vidal-Naquet, G.; Wirsing, M.: Lecture Notes in Computer Science, Vol. 247: 4th Annual Symposion on Theoretical Aspects of Computer Science, Passau, pages 371-382. Springer-Verlag, 1987.