1. For a recent summary see E.H. Thorndike: Proc. of the 1985 Int. Symposium on Lepton and Photon Interactions at High Energies, Eds. M. Konuma and K. Takahashi, p. 406 (1985); R.M. Baltrusaitis et al.: Phys. Rev. Lett. 56 (1986) 2136; C. Bebek et al.: Phys. Rev. Lett. 56 (1986) 1893
2. R. Rückl: CERN report CERN-TH 4013/84 (1984)
3. M. Bauer, B. Stech: Phys. Lett. 152B (1985) 350
4. A.N. Kamal: Phys. Rev. D33 (1986) 1344
5. H.Y. Cheng: Brandeis report BRX TH-194 (1985); L.L. Chau, H.Y. Cheng: Phys. Rev. Lett. 56 (1986) 1655