1. Electrotechnical Laboratory, Umezono, 1-1-4,Tsukuba City, Ibaraki, Japan and Department of Computer Science, Duke University, Durham, NC
2. Department of Computer Science, Duke University, Durham, NC
3. Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor and Department of Computer Science, Duke University, Durham, NC
4. Microsoft Corporation, Menlo Park, CA and Department of Computer Science, Duke University, Durham, NC