A child plays at a playground.
A child is playing on a school playground.
The playground could be a public park, not a school.
exists e1 x1 x2.(at(e1,x2) & n_playground(x2) & Actor(e1,x1) & v_play(e1) & n_child(x1))
exists e1 x1 x2 x3.(on(e1,x2) & n_playground(x2) & of(x2,x3) & n_school(x3) & Actor(e1,x1) & v_play(e1) & n_child(x1))