Patents.
4,679,911; 4,679,913; 4,681,407; 4,682,841; 4,682,842; 4,682,860; 4,682,861; 4,682,862; 4,682,863; 4,684,221; 4,684,223; 4,685,766; 4,685,768; 4,685,774; 4,687,282; 4,687,286; 4,687,302; 4,687,303.