1. United States Patent 5,051,471, September 24, 1991.
2. United States Patent 4,985,498, January 15, 1991.
3. United States Patent 4,957,969, September 18, 1990.
4. United States Patent 4,921,911, May 1, 1990.
5. United States Patent 5,176,751, January 5, 1993.