NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

This summary is based on work supported by the Burroughs Wellcome Fund, Department of Energy, Microsoft Corporation, National Science Foundation (under Grant No. DMS-0109132), and the Sloan Foundation. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsors.

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