A girl rides a horse.
A girl is on top of a horse's back.
If one rides a horse, they are on top of a horse's back.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_horse(x2) & n_girl(x1))
exists p1 x1.(exists x2 x3 x4.(on(x1,x2) & of(x2,x4) & n_back(x4) & of(x4,x3) & n_horse(x3) & n_top(x2)) & n_girl(x1))