Patents.
4,989,959; 4,991,923; 5,009,243; 5,010,770; 5,011,265; 5,013,131; 5,013,133; 5,013,134; 5,013,136; 5,013,139; 5,013,142; 5,013,143; 5,014,709; 5,015,049; 5,015,052; 5,018,803; 5,020,888; 5,020,910; 5,022,726; 5,022,730; 5,022,743.