1. For a recent review of the eight vertex model, see Baxter, R.J.: Exactly solved models in statistical mechanics. London: Academic Press 1982
2. Lieb, E.H., Wu, F.Y.: In: Phase transitions and critical phenomena. Domb, C., Green, M.S. (eds.), Vol. 1. London: Academic Press 1972
3. A more general domain wall model including bending energies has been discussed recently; see Rys, F.S.: Phys. Rev. Lett.51, 849 (1983)
4. Rys, F.S., Helfrich, W.: J. Phys. A15, 599 (1982)
5. Karowski, M., Thun, H.-J., Helfrich, W., Rys, F.S.: J. Phys. A16, 4073 (1983)