A biker rides a motorcycle on a racetrack
A person is riding on a skateboard.
A person does not have to be a biker.  A person either rides a motorcycle or a skateboard.
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & on(x2,x3) & n_racetrack(x3) & n_motorcycle(x2) & n_biker(x1))
exists e1 x1 x2.(on(e1,x2) & n_skateboard(x2) & Actor(e1,x1) & v_ride(e1) & n_person(x1))