1. W.J. Willis: Talk given at the 4th International Colloquium on Photon-Photon Interactions, Paris (1981), CERN preprint 1981, CERN EP/81-45
2. A.P. Contogouris, S. Papadopoulos, C. Papavassiliou: Nucl. Phys.B179, 461 (1981)
3. F. Halzen, D.M. Scott: Madison preprint 1981, MAD/PH/21-1981 and references therein
4. J.F. Owens: Florida State University preprint 1982, FSU-HEP-820315 and references therein
5. E. Anassontzis et al.: CERN preprint 1982, CERN EP/82-26