1. P. Ruján, preceding paper, this issue.
2. I. Peschel and V. J. Emery,Z. Phys. B 43:241 (1981).
3. For a review see P. W. Kasteleyn, inFundamental Problems in Statistical Physics, Vol. 3, E. D. G. Cohen, ed., North-Holland, Amsterdam (1975).
4. J. Kurman, H. Thomas, and G. Müller,Physica A, to be published.
5. R. J. Baxter, inFundamental Problems in Statistical Physics, Vol. 5, E. D. G. Cohen, ed., North-Holland, Amsterdam (1980).