People operating a cement truck
People engaged in a snowball fight.
If you're operating a cement truck, you're really not able to be engaged in a snowball fight at the same time.
exists e1 x1 x2 x3 x4.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_operate(e1) & n_truck(x3) & of(x3,x4) & n_cement(x4) & (x1 = x2) & n_people(x1))
exists e1 x1 x2 x3.(in(e1,x2) & n_fight(x2) & of(x2,x3) & n_snowball(x3) & Actor(e1,x1) & v_engage(e1) & n_people(x1))