a skateboarder grinds a rail.
A skateboarder is wearing a helmet.
Not all skateboarder is wearing a helmet.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_rail(x2) & n_skateboarder(x1))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_wear(e1) & n_helmet(x2) & n_skateboarder(x1))