1. Slichter, C.P.: Principles of magnetic resonance. In: Springer Series in Solid-State Sciences, Vol. 1, 2nd Edn. Berlin, Heidelberg, New York: Springer-Verlag 1978
2. Roldan, J.M.R., McCoy, B.M., Perk, J.H.H.: Preprint ITP-SB-45, State University of New York, Stony Brook. Physica A (to be published)
3. The idea of evaluating spin commutators by computer is not new, ef. e.g. Morita, T.: J. Math. Phys.12, 2062 (1971)
4. Oitmaa, J., Plischke, M., Winchester, T.A.: Phys. Rev. B29, 1321 (1984)
5. Platz, O., Gordon, R.G.: Phys. Rev. Lett.30, 264, (1973)