1. For an overview, see D. Drechsel et al., Working Group Summary, nucl-th/9712013, and references therein.
2. H. Ankin et al., Phys. Lett. B 336 (1994) 313; E. Bruins et al., Phys. Rev. Lett. 75 (1995) 21; M. Meyerhoff et al., Phys. Lett. B 327 (1994) 201; J. Jourdan et al., A1 Collaboration at MAMI, to be published.
3. F. Klein, in: Proc. of PANIC'96, Williamsburg, 1996; H. Schmieden, in: Proc. of SPIN'96, Amsterdam, 1996; S. Platchkov et al., Nucl. Phys. A 510 (1990) 740.
4. N. Isgur, G. Karl, Phys. Rev. D 18 (1978) 4187; D 19 (1979) 2653; D 20 (1979) 1191; R. Koniuk, N. Isgur, Phys. Rev. D 21 (1980) 1868.
5. A.W. Thomas, Adv. in Nucl. Phys. 13 (1984) 1; S. Théberge, G.A. Miller, A.W. Thomas, Can. J. Phys. 60 (1982) 59; S. Théberge, A.W. Thomas, Nucl. Phys. A 393 (1983) 252.