1. B. Richter, Nucl. Instr. and Meth. 136: 47 (1976).
2. U. Amaldi, Phys. Letters 61B: 313 (1976).
3. M. Tigner, Nuovo Cimento 37: 313 (1965).
4. U. Amaldi, in “Proc. of the 1979 Int. Symposium on lepton and photon interactions”, Fermi-lab, August 23–29, T.B.W. Kirk and H.D.I. Abarbanel, eds., FNAL, Batavia Ill (1979) p. 314.
5. U. Amaldi, in: “Proc. of the Workshop on physics at future accelerators”, La Thuile (Italy) and Geneva (Switzerland), 7–13 Jan., 1987, J. Mulvey, ed. f CERN, Geneva (1987) Vol. 1, p. 323.