a skateboarder grinds a curb.
A boy skates on his skateboard.
Skateboarders are not always a boy.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_curb(x2) & n_skateboarder(x1))
exists e1 x1 x2.(on(e1,x2) & n_skateboard(x2) & of(x2,x1) & n_male(x1) & Actor(e1,x1) & v_skate(e1) & n_boy(x1))