Aarhus Univ., Comput Science Dep., DAIMI PB-120, 1980.
Also in: Gruska, J.; Chytil, M.: Lecture Notes in Computer Science, Vol. 118: Mathematical Foundation of Computer Science, Proc. of the 10th Symp., Strbske Pleso 1981, pages 327-338. Berlin, New York: Springer-Verlag, 1981.