A cowboy on the back of a galloping horse.
A farmer is looking at the horse.
The subject cannot be a cowboy and a farmer.
exists x1 x2 x3 x4.(n_back(x2) & a_topic(x1) & on(x1,x2) & of(x2,x3) & n_horse(x3) & of(x3,x4) & n_galloping(x4) & n_cowboy(x1))
exists e1 x1 x2.(n_horse(x2) & at(e1,x2) & Actor(e1,x1) & v_look(e1) & n_farmer(x1))