1. Ashtekar, A., Romano, J. D., and Tate, R. S. (1989) Phys. Rev. D40, 2572.
2. Ashtekar, A., and Stachel, J. (1991) Conceptual Problems of Quantum Gravity, Birkhäuser, Boston.
3. Band, W. (1942) Phys. Rev. 61, 698.
4. Barbour, J., and Pfister, H. (1995) Macks Principle: From Newton’s Bucket to Quantum Gravity, Birkhäuser, Boston.
5. Bergmann, P. G. (1962) General relativity of theory, in S. Flügge (ed.), Encyclopedia of Physics, Springer, Berlin et. al., vol. II.