1. Department of Mathematics, Princeton University, Princeton, NJ 08544 USA.
2. Princeton Neuroscience Institute, Princeton University, Princeton, NJ 08540 USA.
3. Departments of Computer Science and Psychology, Princeton University, Princeton, NJ 08544 USA.
4. Google Research, Mountain View, CA 94043 USA.
5. Department of Computer Science, Worcester Polytechnic Institute, Worcester, MA 01609 USA.
6. School of Computing Science at Simon Fraser University, Burnaby, BC V5A 1S6, Canada.
7. Amazon, Seattle, WA 98109 USA Work done prior to being affiliated with Amazon.