1. WellsJrR. O., Function theory on differentiable manifolds, in Contributions to Analysis, Academic Press, New York, 1974.
2. PenroseR. and RindlerW., Spinors and Spacetime, vol. 2, Cambridge University Press, Cambridge, 1986.
3. RobinsonI. and TrautmanA., Cauchy-Riemann structures in optical geometry, in R.Rufini (ed.), Proc. Fourth Marcel Grossman Meeting on General Relativity, Elsevier, Amsterdam, 1986.
4. Robinson, I. and Trautman, A., Optical geometry, Warsaw University preprint IFT/7/87 (1987).
5. TafelJ., Lett. Math. Phys. 10, 33 (1985).