Patents.
4,488,775; 4,497,540; 4,498,009; 4,500,163; 4,500,175; 4,501,470; 4,501,961; 4,502,756; 4,504,110; 4,504,111; 4,504,121; 4,504,124; 4,504,122; 4,509,857; 4,504,125; 4,504,123; 4,504,128; 4,505,535; 4,505,536; 4,505,550; 4,505,553; 4,507,776.