A boy on a swing.
A boy on the monkeybars.
The boy can't be on a swing and the monkeybars at the same time.
exists x1 x2.(a_topic(x1) & on(x1,x2) & n_swing(x2) & n_boy(x1))
exists x1 x2.(n_monkeybar(x2) & a_topic(x1) & on(x1,x2) & n_boy(x1))