A person riding a dirt bike.
a person is on a vehicle
person riding a dirt bike is just as person is on a vehicle
exists e1 x1 x2 x3 x4.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_bike(x3) & of(x3,x4) & n_dirt(x4) & (x1 = x2) & n_person(x1))
exists p1 x1.(exists x2.(on(x1,x2) & n_vehicle(x2)) & n_person(x1))