A skateboarder performs a trick on a ramp.
The skateboarder is riding a skateboard.
If a skateboarder performs a trick, then he must be on a skateboard.
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_perform(e1) & on(x2,x3) & n_ramp(x3) & n_trick(x2) & n_skateboarder(x1))
exists e1 x1 x2.(n_skateboarder(x1) & Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_skateboard(x2))