Patents.
4,165,180; 4,156,206; 4,158,201; 4,166,769; 4,166,917; 4,173,474; 4,174,179; 4,174,881; 4,175,750; 4,175,823; 4,178,064; 4,178,079; 4,179,182; 4,181,430; 4,181,399; 4,181,435; 4,183,669; 4,184,747; 4,184,767; 4,185,893; 4,188,091; 4,188,092; 4,184,737; 4,188,519; 4,188,542; 4,189,211; 4,189,212; 4,189,213; 4,190,323; 4,190,324; 4,190,353.