1. For a recent review, see A. O. Barut, ?Brief history and recent developments in electron theory and quantum electrodynamics,? inThe Electron 1990, D. Hestenes and A. Weingartshofer, eds. (Kluwer Academic, Dordrecht, 1991), pp. 105?148.
2. J. Rawnsley, ?Exact symplectic structures and a classical model for the Dirac electron,?Lett. Math. Phys. 24, 331 (1992).
3. W. A. Rodrigues, Jr., J. Vaz, Jr., and E. Recami, ?Aboutzitterbewegung and electron structure,? Catania preprint, 1993.
4. A. O. Barut, ?Excited states ofzitterbewegung,?Phys. Lett. B 237, 436 (1990).
5. For a list of references on theories of spinning particles, see A. O. Barut and M. G. Cruz, ?Classical relativistic spinning particle with anomalous magnetic moment: The precession of spin,?J. Phys. A: Math. Gen. (1993) (in press) (ICTP, Trieste preprint IC/93/104, 1993).