1. Gasiorowicz, S., Geffen, D. A.: Rev. Mod. Phys.41, 531 (1969).
2. Macfarlane, A. J., Sudbery, A., Weisz, P. H.: Commun. math. Phys.11, 77 (1968).
3. Barnes, K. J., Dittner, P.: To be published.
4. Gell-Mann, M.: The eightfold way. California Institute of Technology Report CTSL-20 (1961) unpublished; reproduced in: The eightfold way, M. Gell-Mann and Y. Ne'eman. New York: Benjamin Inc. 1964.