1. S. Weinberg, Paper Contributed to the 19th International Conference on High Energy Physics, Tokyo (1978).
2. C. Y. Prescott, W. B. Atwood, R. L. A. Cottrell et al., Phys. Lett., B77, 347 (1978).
3. L. M. Barkov and M. S. Zolotorev, Pis'ma Zh. Eksp. Teor. Fiz.,27, 379 (1978);28, 544 (1978).
4. S. Weinberg, Phys. Rev., D5, 1412 (1972).
5. I. Bernabeu, Preprint TH-1751, CERN (1973).