1. For a review and a list of references, see,e. g.,Chan Hong-Mo: CERN preprint TH. 1057 or theProc. Roy. Soc., to be published (1969).
2. Z. Koba andH. B. Nielsen: Niels Bohr Institute preprint (Feb., 1969).
3. Z. Koba andH. B. Nielsen: Niels Bohr Institute preprint (July, 1969).
4. D. B. Fairlie andK. Jones: University of Durham preprint (1969).
5. See,e.g.,K. Bardakçi andH. Ruegg: Berkeley preprint (1968).