1. Trautman, A., Nature phys. Sci., 242, 7 (1973).
2. Isham, C. J., Salam, A., and Strathdee, J., Nature phys. Sci., 244, 82 (1973).
3. Sivaram, C., and Sinha, K. P., Lett. Nuovo Cim., 8, 324 (1973).
4. Hawking, S. W., and Penrose, R., Proc. R. Soc. Lond., A 314, 529 (1970).
5. Wheeler, J. A., in Relativity Groups and Topology, (edit. by Dewitt, C. and B.), 328 (Gordon and Breach, New York, 1964).