Patents.
4,152,767; 4,158,507; 4,215,938; 4,272,195; 4,313,647; 4,339,178; 4,342,502; 4,344,675; 4,344,676; 4,344,709; 4,345,153; 4,345,817; 4,346,295; 4,346,990; 4,347,530; 4,349,901; 4,350,149; 4,350,410.