A policeman on a horse surrounded by onlookers.
the policeman is at the derby
the policeman is at a derby not on a horse.
exists e1 x1 x2 x3 x4.(a_topic(x1) & on(x1,x2) & Actor(e1,x4) & n_onlooker(x4) & Theme(e1,x3) & v_surround(e1) & (x2 = x3) & n_horse(x2) & n_policeman(x1))
exists p1 x1 x2.(n_derby(x2) & n_policeman(x1) & at(x1,x2))