[1] UCL, Dept Comp Sci, London WC1E 6BT, England
[2] Univ Cambridge, Cambridge judge Business Sch, Ctr Risk Studies, Cambridge CB21 AG, England
[3] Santa Fe Inst, Santa Fe, NM 87501 USA
[4] Univ Oxford, Oxford Martin Sch, Inst New Econ Thinking, Oxford OX2 6ED, England
[5] Univ Oxford, Math Inst, Oxford OX2 6ED, England
[6] Dartmouth Coll, Dept Comp Sci, Hanover, NH 03755 USA
[7] Dartmouth Coll, Dept Math, Hanover, NH 03755 USA