a skateboarder grinds the rail.
A skateboarder is doing tricks at the skate park.
Grinding the rail does not imply they are at a skate park.
exists e1 x1 x2.(n_rail(x2) & Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_skateboarder(x1))
exists e1 x1 x2 x3 x4.(n_park(x3) & of(x3,x4) & n_skate(x4) & at(e1,x3) & Theme(e1,x2) & Actor(e1,x1) & v_do(e1) & n_trick(x2) & n_skateboarder(x1))