1. R. Rückl: Habilitationsschrift, University of Munich, 1983
2. M. Wirbel: Prog. Part. Nucl. Phys. 21 (1988) 33
3. M. Bauer, B. Stech and M. Wirbel: Z. Phys. C 34 (1987) 103; see also, M. Wirbel, B. Stech and M. Bauer: Z. Phys. C 29 (1985) 637; M. Bauer and M. Wirbel: Z. Phys. C 42 (1989) 671
4. M. Neubert, V. Rieckert, B. Stech and Q. P. Xu: in Heavy Flavours, ed. by A. J. Buras and M. Lindner, World Scientific, Singapore, 1992
5. T.E. Browder, K. Honscheid and S. Playfer: Report CLNS 93/1261, to appear in B Decays, 2 nd Edition, ed. by S. Stone. World Scientific, Singapore