1. J. D. Bjorken:Phys. Rev.,148, 1467 (1966).
2. H. Harari:Ann. of Phys.,63, 432 (1971).
3. R. Dolen, D. Horn andC. Schmidt:Phys. Rev.,166, 1768 (1968).
4. For a recent review, cf.P. Di Vecchia andS. Fubini:Erice Lectures, 1972 (CERN preprint TH 1542 (1972)).
5. Cf.M. Flato andD. Sternheimer:Compt. Rend.,263, 935 (1966);M. Flato, J. Simon andD. Sternheimer:Ann. of Phys.,63, 432 (1971);D. J. Gross andJ. Wess:Phys. Rev. D,2, 753 (1970);B. Schroer:Lett. Nuovo Cimento,2, 873 (1971).