1. E. J. Saletan andA. H. Cromer:Theoretical Mechanics (New York, N. Y., 1971), p. 83, 233.
2. F. González-Gascón:Lett. Nuovo Cimento,19, 366 (1977);20, 54 (1977).
3. Einstein’s convention for repeated indices will be used throughout this paper.
4. Y. Choquet-Bruhat:Géométrie différentielle et systèmes extérieures (Paris, 1968), p. 151.
5. Henceforth the adjective «local» will in general be omitted, but the generally local character of our considerations should be understood.