1. L. Puppe and J. Weisser, US Pat., 4 439 409 (1984).
2. S.I. Zones, Eur. Pat. Appl., EP 231 019 (1987).
3. M.K. Rubin and P. Chu, US Pat. 4 954 325 (1990).
4. R.P.L. Absil, S. Han, D.O. Marler and D.S. Shihabi, US Pat. 4 962 257 (1990).
5. C.T.W. Chu, T.F. Degnan and B.K. Huh, US Pat., 4 982 033 (1991).