a skateboarder grinds a rail.
An expert skateboarder rides a rail.
Just because a skateboarder grinds a rail doesn't imply the skateboarder is an expert.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_rail(x2) & n_skateboarder(x1))
exists e1 x1 x2 x3.(Theme(e1,x3) & Actor(e1,x1) & v_ride(e1) & n_rail(x3) & n_skateboarder(x1) & of(x1,x2) & n_expert(x2))