1. Dept. of Computer Science, The Pensylvania State University, University Park, PA 16802. e-mail: dima@cs.psu.edu
2. Department of Computer Science, University of Bonn, 53117 Bonn, and the International Computer Science Institute, Berkeley, California. e-mail: marek@cs.bonn.edu
3. AT&T Bell Laboratories, Murray Hill, NJ 07974. e-mail: amo@research.att.com