a skateboarder grinds a rail.
A skateboarder is at a competition.
A skateboarder who grinds a rail is not necessarily at a competition.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_rail(x2) & n_skateboarder(x1))
exists p1 x1.(exists x2.(at(x1,x2) & n_competition(x2)) & n_skateboarder(x1))