a horse and rider jump over a fence.
A person horseback riding.
For a horseback rider to ride the horse, they must be horseback riding.
exists s1 x1 x2 x3 x4.(a_topic(s1) & subset_of(x2,s1) & over(x2,x4) & n_fence(x4) & n_jump(x2) & of(x2,x3) & n_rider(x3) & subset_of(x1,s1) & n_horse(x1))
exists x1 x2.(a_topic(x1) & n_horseback(x1) & Time(x1,x2) & n_riding(x2) & n_person(x1))