1. See for instanceS. Y. Chu, G. Epstein, P. Kaus, R. C. Slansky andF. Zachariasen:Phys. Rev.,175, 2098 (1968) and references contained therein.
2. M. Froissart:Theoretical Physics (Vienna, 1963), p. 379.
3. R. Arnold:Phys. Rev. Lett.,14, 657 (1965).
4. See for instanceM. Jacob:Lecture Notes at the VIII Schladming Winter School, 1969, CERN preprint Th. 1010 (1969). See also ref. (5).
5. E. Predazzi:Lecture Notes at the Theoretical Summer Institute, University of Colorado (1969) (to be published) and Indiana University preprint (1969).