Acknowledgements

I am grateful to many people for discussions, including C. Bennett, J. Crutchfield, D. Friedan, P. Gacz, E. Jen, D. Lind, O. Martin, A. Odlyzko, N. Packard, S. Shenker, W. Thurston, T. Toffoli and S. Willson. I am particularly grateful to J. Milnor for extensive discussions and suggestions.

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