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3. R. Kikuchi, Prog. Theor. Phys. Suppl. 35, 1 (1966).
4. For recent development of the CVM and the PPM, see Prog. Theor. Phys. Suppl. 115 (1994).
5. P. Bak, C. Tang, and K. Wiesenfeld, Phys. Rev. Lett. 59, 381 (1987).