Department of Computer Science, Columbia University, New York, NY 10027, USA. abk2001@cs.columbia.edu|Department of Computer Science, Columbia University, New York, NY 10027, USA. abk2001@cs.columbia.edu|Department of Computer Science, Columbia University, New York, NY 10027, USA. abk2001@cs.columbia.edu|Department of Computer Science, Columbia University, New York, NY 10027, USA. abk2001@cs.columbia.edu|Department of Computer Science, Columbia University, New York, NY 10027, USA. abk2001@cs.columbia.edu|Department of Computer Science, Columbia University, New York, NY 10027, USA. abk2001@cs.columbia.edu