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2. Müller, H., Lücke, M. and Kamps, M. (1989) Europhys. Lett. 10, 451.
3. Tsameret, A. and Steinberg, V. (1991) Europhys. Lett. 14, 331.
4. Cross, M.C. (1988) Phys. Rev. A38, 3593.
5. Deissler, R.J. (1989) Physica 25D, 233.