A person riding a bmx bike on a dirt course.
A person is sitting on a bike.
we don't know the person is sitting on the bike.
exists e1 x1 x2 x3 x4 x5 x6.(a_topic(x1) & on(e1,x5) & n_course(x5) & of(x5,x6) & n_dirt(x6) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_bike(x3) & of(x3,x4) & n_bmx(x4) & (x1 = x2) & n_person(x1))
exists e1 x1 x2.(on(e1,x2) & n_bike(x2) & Actor(e1,x1) & v_sit(e1) & n_person(x1))