A skateboarder riding a rail.
A skateboarder is demonstrating tricks for a video.
Just because a skateboarder is riding a rail, it doesn't necessarily mean that the skateboarder is demonstrating tricks to anyone, or doing so for a video.
exists e1 x1 x2 x3.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_rail(x3) & (x1 = x2) & n_skateboarder(x1))
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_demonstrate(e1) & for(x2,x3) & n_video(x3) & n_trick(x2) & n_skateboarder(x1))