1. C. Cronström:Phys. Lett. B,90, 267 (1980).
2. V. Fateev, A. Schwarz andY. Tyupkin: preprint FIAN-155 (Moscow, 1980).M. S. Dubovikov andA. V. Smilga:Proceedings of the International Seminar «Group Methods in Field Theory» (Moscow, 1979);Nucl. Phys. B,185, 109 (1981);
3. M. A. Shifman:Nucl. Phys. B,173, 13 (1980);
4. M. Azam:Phys. Lett. B,101, 401 (1981);
5. K. Itabashi:Prog. Theor. Phys.,65, 1423 (1981);