A girl is sitting on a guy's lap at a party.
The guy is sitting in the girl's lap.
Either the girl is sitting on a guy's lap or the guy is sitting on the girl's lap.  Both cannot be happening at the same time
exists e1 x1 x2 x3 x4.(at(e1,x4) & n_party(x4) & on(e1,x3) & n_lap(x3) & of(x3,x2) & n_guy(x2) & Actor(e1,x1) & v_sit(e1) & n_girl(x1))
exists e1 x1 x2 x3.(n_lap(x3) & of(x3,x2) & n_girl(x2) & n_guy(x1) & in(e1,x3) & Actor(e1,x1) & v_sit(e1))