1. Aleksandrov, P. S. (1956).Combinatorial Topology, Greylock, Rochester, New York, Volume I.
2. Bombelli, L., Lee, J., Meyer, D., and Sorkin, R. D. (1987).Physical Review Letters,59, 521?524, and references therein.
3. Bourbaki, N. (1966).General Topology, Hermann, Paris, Chapter I, � 1, Problem 2.
4. Bourbaki, N. (1968).Theory of Sets, Hermann, Paris, Chapter III, � 7.
5. Duke, D. W., and Owens, I. F., eds. (1985).Advances in Lattice Gauge Theory, World Scientific, Singapore.