 Complete the following phrase:  Jackie Stewart, three-time champion and Chairman of the GPDA, used his position to push for improved safety and track facilities, including safety barriers and mandatory seat belts. This caused some opposition with other drivers, most notably Jacky Ickx, Stewarts opponent on the track at the time. Stewarts influence led to boycotts of some races, including the 1971 Belgian Grand Prix. Despite his efforts, drivers continued to be killed, including Stewarts own teammate, François Cevert, at the 