1. See, e.g., C. W. Misner, K. S. Thorne, and J. A. Wheeler,Gravitation (Freeman, New York, 1973), Sec. 21.7.
2. J. R. Klauder,Phys. Rev. D,19, 2349 (1979).
3. P. A. M. Dirac,The Principles of Quantum Mechanics, 3rd edn. (Oxford University Press, Oxford, 1947), p. 114.
4. J. Sniatycki,Geometric Quantization and Quantum Mechanics (Springer, New York, 1980).
5. F. Bayen, M. Flato, C. Fronsdal, A. Lichnerowicz, and D. Sternheimer,Ann. Phys. (New York) 111, 61 (1978).