1. Guglielmetti, L.; Basel. US Patent 4267343, 1978.
2. Duenndnbberger, M.; Adolf, E. S.; Maeder, E. US Patent 2995564, 1961.
3. Liechti, T.; Adolf, E. S. US Patent 3095421, 1963.
4. Peter, L.; Binninged; Adolf, E. S.; Basel; Erwin, M. US Patent 3127416, 1964.
5. British patent specification No. 940770, 1963.