Children are playing on a playground.
The children are in class.
The children cannot be in class and playing on the playground simultaneously.
exists e1 x1 x2.(on(e1,x2) & n_playground(x2) & Actor(e1,x1) & v_play(e1) & n_child(x1))
exists p1 x1.(n_child(x1) & exists x2.(in(x1,x2) & n_class(x2)))