Department of Computer Science, University of Kaiserslautern, D-67653 Kaiserslautern, P.O. Box 3049, Germany. a_scheid@cs.uni-kl.de|Department of Computer Science, University of Kaiserslautern, D-67653 Kaiserslautern, P.O. Box 3049, Germany. a_scheid@cs.uni-kl.de