Athletes running in a race.
Nobody is running.
Athletes cannot be running in a race if nobody is running.
exists e1 x1 x2 x3.(a_topic(x1) & in(e1,x3) & n_race(x3) & Actor(e1,x2) & v_run(e1) & (x1 = x2) & n_athlete(x1))
-exists e1 x1.(Actor(e1,x1) & v_run(e1) & n_person(x1))