Man on a dirt bike
There is nobody on the dirt bike.
If there is nobody on a dirt bike then there is not a man on a dirt bike.
exists x1 x2 x3.(a_topic(x1) & on(x1,x2) & n_bike(x2) & of(x2,x3) & n_dirt(x3) & n_man(x1))
exists p1 x1 x3 x4.(n_bike(x3) & of(x3,x4) & n_dirt(x4) & -exists x2.((x1 = x2) & on(x2,x3) & n_person(x2)))