A man is operating a piece of machinery.
A man working on a vehicle
If you are working on machinery it probably is broken and not operating.
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_operate(e1) & of(x2,x3) & n_machinery(x3) & n_piece(x2) & n_man(x1))
exists e1 x1 x2 x3.(a_topic(x1) & on(e1,x3) & n_vehicle(x3) & Actor(e1,x2) & v_work(e1) & (x1 = x2) & n_man(x1))