1. Kaufman, C. W., Fd Technol., 1951, 5, 154–9.
2. Schmid, H., German Federal Republic Patent Application 8 116265, 1981.
3. Stolz, R. P., Ouellette, N. P., Linthorst, J. M. and Herlihy, T. V., US Patent 4 303 686, 1981.
4. Magnolato, D., US Patent, 4278696, 1981.
5. Brambilla, E., US Patent, 4 317 841, 1982.