Patents.
4,498,771; 4,513,434; 4,521,068; 4,522,466; 4,541,688; 4,541,716; 4,541,720; 4,542,299; 4,542,954; 4,542,955; 4,542,961; 4,542,963; 4,544,228; 4,545,654; 4,544,229; 4,544,266; 4,545,678; 4,546,244.