a skateboarder grinds the rail.
A surfer catches a wave.
A skateboarder is different than a surfer
exists e1 x1 x2.(n_rail(x2) & Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_skateboarder(x1))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_catch(e1) & n_wave(x2) & n_surfer(x1))