A person on a bmx bike.
A person riding a bike over dirt obsticals.
A person on a bike does not mean they are riding over dirt obstacles.
exists x1 x2 x3.(a_topic(x1) & on(x1,x2) & n_bike(x2) & of(x2,x3) & n_bmx(x3) & n_person(x1))
exists e1 x1 x2 x3 x4 x5.(a_topic(x1) & over(e1,x4) & n_obstical(x4) & of(x4,x5) & n_dirt(x5) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_bike(x3) & (x1 = x2) & n_person(x1))