Rollerblades sliding on a railing.
The rollerblades are by a railing.
Both sentence are talking about rollerblades.
exists e1 x1 x2 x3.(a_topic(x1) & on(e1,x3) & n_railing(x3) & Actor(e1,x2) & v_slide(e1) & (x1 = x2) & n_rollerblade(x1))
exists p1 x1.(n_rollerblade(x1) & exists x2.(by(x1,x2) & n_railing(x2)))