a skateboarder grinds a rail.
A skateboarder is performing.
A skateboarder who grinds a rail is not necessarily performing.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_rail(x2) & n_skateboarder(x1))
exists e1 x1.(Actor(e1,x1) & v_perform(e1) & n_skateboarder(x1))