[1] Carnegie Mellon Univ, Sch Comp Sci, 5000 Forbes Ave, Pittsburgh, PA 15213 USA
[2] Columbia Univ, Dept Comp Sci, 500 West 120 St,Room 450,MC0401, New York, NY 10027 USA
[3] Stanford Univ, Gates Comp Sci Bldg,353 Jane Stanford Way, Stanford, CA 94305 USA