a skateboarder grinds the rail.
The boy is playing on a skateboard.
Not all skateboarders are a boy.
exists e1 x1 x2.(n_rail(x2) & Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_skateboarder(x1))
exists e1 x1 x2.(n_boy(x1) & on(e1,x2) & n_skateboard(x2) & Actor(e1,x1) & v_play(e1))