Univ. de Paris-Sud, Centre d'Orsay, Laboratoire de Recherche en Informatique, U.A. au CNRS 410 ``Al Khowarizmi'', Bat. 490, 91405 Orsay Cedex (France), Rapport de Recherche No. 297, 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.