1. P. A. M. Dirac:Rev. Mod. Phys.,21, 392 (1959).
2. L. Bel:Ann. Inst. H. Poincaré,14 A, 189 (1971).
3. See,e.g.,E. C. G. Sudarshan:Action-at-a-distance (preprint, Austin, Tex., 1970).
4. J. M. Souriau:Structures des systèmes dynamiques (Paris, 1970).
5. B. Kostant:Lectures in Modern Analysis and Applications, Vol.3, edited byC. T. Taam (Berlin, 1970).