 He was assistant professor from 1963 to 1966 at the University of Michigan, and then joined the faculty of the E.E.Dept at the University of California at Berkeley, where he was an associate professor from 1966 to 1971, and a full professor from 1971 to 1994.In the 1960s, he worked with Sheila Greibach, Gene Rose, Ed Spanier, and Joe Ullian in a research group formed and led by Seymour Ginsburg, dedicated to formal language theory and the foundations of Computer Science.