1. U.S.Patent No. 2,438,019 (1948)
2. U.S.Patent No. 3,055,738 (1962)
3. West-German Patent Application OSL No. 2,128,753 (1971)
4. West-German Patent Application OSL No. 2, 244,264 (1973)
5. Japanese Published Unexamined Patent Application No. 15,318 (1978)