1. We follow the conventions in the textbook ofK. Nishijima:Fields and Particles (W. A. Benjamin, Inc., New York, N. Y., 1969), in whichx 4=ix0 andh=c=1.
2. L. P. Horwitz andY. Lavie:Phys. Rev. D,26, 819 (1982);D. M. Greenberger:J. Math. Phys. (N. Y.),11, 2329, 2341 (1970);15, 395, 406 (1974).L. P. Horwitz andC. Piron:Helv. Phys. Acta,46, 316 (1973);L. P. Horwitz, C. Piron andF. Reuse:Helv. Phys. Acta,48, 546 (1975);C. Piron andR. Reuse:Helv. Phys. Acta,51, 146 (1978);F. Reuse:Helv. Phys. Acta,51, 157 (1978).
3. J. R. Fanchi:Phys. Rev. D,20, 3108 (1979), and references therein.
4. Y. Nambu:Prog. Theor. Phys.,5, 82 (1950).
5. V. A. Fock:Phys. Z. Sowjetunion,12, 404 (1937);E. C. G. Stuckelberg:Helv. Phys. Acta,14, 322 (1941);14, 23 (1942);R. P. Feynman:Phys. Rev.,80, 440 (1950);84, 108 (1951);J. Schwinger:Phys. Rev.,82, 664 (1951);W. C. Davidon:Phys. Rev.,97, 1131 (1955);G. Szamosi:Nuovo Cimento,20, 1090 (1961);H. Enatsu:Prog. Theor. Phys.,30, 236 (1963);J. H. Cooke:Phys. Rev.,166, 1293 (1968);L. Hostler:J. Math. Phys. (N. Y.),21, 2461 (1980);22, 2307 (1981).