The girl is sitting on a swing.
A girl is on a swing.
If you are sitting on a swing than you are clearly on a swing.
exists e1 x1 x2.(n_girl(x1) & on(e1,x2) & n_swing(x2) & Actor(e1,x1) & v_sit(e1))
exists p1 x1.(exists x2.(on(x1,x2) & n_swing(x2)) & n_girl(x1))