Patents.
5,048,937; 5,050,967; 5,054,924; 5,056,920; 5,069,545; 5,090,801; 5,111,055; 5,130,533; 5,150,170; 5,170,063; 5,170,268; 5,171,981; 5,173,748; 5,175,737; 5,175,757; 5,185,749; 5,187,756; 5,192,863; 5,194,980.