1. For some recent reviews see e.g. R M Wald, gr-qc/9710068
2. R Penrose, in Black holes and relativistic stars edited by R M Wald (University of Chicago Press, 1998)
3. P S Joshi, in Singularities, black holes and comsic censorship (IUCAA Publications, Pune, 1997) gr-qc/9702036
4. A Krolak, gr-qc/9910108
5. T P Singh, gr-qc/9805066