1. Penrose, R., in J.Klauder (ed.),Magic Without Magic, Freeman, San Francisco, 1972.
2. Witten, E.,Nuclear Phys. 311B, 46 (1989).
3. Horowitz, G. T.,Comm. Math. Phys. 125, 417 (1989).
4. Witten, E., in C. N.Yang and M. L.Ge (eds.),Braid Group, Knot Theory and Statistical Mechanics, World Scientific, Singapore, 1989.
5. Witten, E.,Comm. Math. Phys. 117, 353 (1988).