1. J. Blümlein, J. Feltesse, M. Klein, in: Proc. of the ECFA Large Hadron Collider Workshop, Aachen 1990, eds. G. Jarlskog and D. Rein, Vol. 2 (CERN, Geneva, 1991), p. 830, CERN 90-10, ECFA 90-133; M. Klein, in: Proc. of the 1991 HERA Physics Workshop, eds. W. Buchmüller, G. Ingelman, Vol. 1, p. 71
2. J. Blümlein, M. Klein: Nucl. Instr. Meth. A329 (1993) 112
3. M. Derrick et al., ZEUS collaboration Phys. Lett. B316 (1993) 412; I. Abt et al., H1 collaboration, Nucl. Phys. B407 (1993) 515
4. For a recent review see: H. Spiesberger, et al., in: Proc. of the 1991 HERA Physics Workshop, eds. W. Buchmüller, G. Ingelman, Vol. 2, p. 798, and references therein
5. W. Beenakker, F. Berends, W. van Neerven: Proceedings of the Ringberg Workshop 1989, ed. J.H. Kühn, (Springer, Berlin, 1989), p. 3; J. Blümlein: Z. Phys. C47 (1990) 89