Patents.
5,148,235; 5,170,048; 5,172,252; 5,173,908; 5,191,206; 5,192,869; 5,192,981; 5,198,653; 5,198,895; 5,198,912; 5,201,220; 5,202,775; 5,206,499; 5,210,624; 5,214,685; 5,216,257; 5,218,471; 5,218,480; 5,221,834; 5,221,835; 5,221,839; 5,221,975; 5,221,990; 5,225,918; 5,227,915; 5,229,597; 5,239,183; 5,241,179; 5,241,419; 5,243,448; 5,243,466; 5,243,583.