1. United States Patent 6,572,965, June 3, 2003.
2. United States Patent 6,534,137, March 18, 2003.
3. United States Patent 5,670,254, September 23, 1997.
4. United States Patent 5,491,013, February 13, 1996.
5. United States Patent 5,300,360, April 5, 1994.