1. I. G. Halliday: Imperial College preprint ICTP /67/26, September 1968.
2. We have heard of work byG. F. Chew andA. Pignotti similar to that in (1), reported at theVienna Conference, August 1968, but have seen no written form of it.G. Chew andA. Pignotti: Berkeley preprint, UCRL-18275, June 1968.
3. I. T. Drummond: Cambridge preprint, March 1968.
4. V. V. Sudakov:Žurn. Ėksp. Teor. Fiz.,30, 87 (1956).
5. I. G. Halliday andL. M. Saunders: Imperial College preprint ICTP/67/34, August 1968.