1. Altarelli G, Cabibbo N and Maiani L 1976Phys. Lett. B57 277
2. Altarelli G, Cabibbo N and Maiani L 1976Nucl. Phys. B88 285
3. Babu P 1965Phys. Rev. Lett. 14 166
4. Bajaj J K and Kapoor A K 1977Phys. Rev. (to be published)
5. Branco G, Mohapatra R N, Haggiwara T and Sidhu D B 1976Phys. Rev. D13 680