A person rides a dirt bike.
The dirt biker is very good.
Not all people who can ride dirt bikes are very good. They could be very bad dirt bike riders.
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_bike(x2) & of(x2,x3) & n_dirt(x3) & n_person(x1))
exists s1 x1 x2.(n_biker(x1) & of(x1,x2) & n_dirt(x2) & r_very(s1) & Theme(s1,x1) & a_good(s1))