A boy standing on a tricycle.
Nobody is standing.
Either a boy is standing or nobody is standing.
exists e1 x1 x2 x3.(a_topic(x1) & on(e1,x3) & n_tricycle(x3) & Actor(e1,x2) & v_stand(e1) & (x1 = x2) & n_boy(x1))
-exists e1 x1.(Actor(e1,x1) & v_stand(e1) & n_person(x1))