tended abstract of this work appears in the proceedings of the 16th Symposium on Theoretical Aspects of Computer Science (STACS’99), Springer, LNCS, held in Trier, Fed. Rep. of Germany, March 4–6, 1999. †Supported by a Feodor Lynen fellowship of the Alexander von Humboldt-Stiftung, Bonn, and the Center for Discrete Mathematics, Theoretical Computer Science and Applications (DIMATIA), Prague. 1