1. Department of Mathematics, Duke University, Durham, North Carolina, U.S.A.; IST Austria (Institute of Science and Technology Austria), Klosterneuburg, Austria
2. Departments of Computer Science and Mathematics, Duke University, Durham, North Carolina, U.S.A.; IST Austria (Institute of Science and Technology Austria), Klosterneuburg, Austria
3. Departments of Computer Science and of Mathematics, Stanford University, Stanford, Calif., U.S.A.
4. Department of Computer Science, Duke University, Durham, North Carolina, U.S.A.; IST Austria (Institute of Science and Technology Austria), Klosterneuburg, Austria