1. For a general review and further refs. see B.H. Wiik, Proceedings of the twenty-first Scottish Universities Summer School in Physics, ed. K. Bowler and D. Sutherland, SUSSP (1980).
2. R. Cashmore, these proceedings
3. For a recent review see F. Sciulli, XX International Conference, Madison, Wisconsin (1980), AIP Conference Proceedings No. 68. 1278.
4. M Jacob and R. Horgan, 1980 CERN/DESY School of Physics, CERN 81-04.
5. H. Fritzsch, M. Gell-Mann and H. Leutwyler, Phys. Lett. 47B (1973) 365.