A firefighter is on the roof.
Nobody on the roof
If a firefighter is on the roof then someone is on the roof, not nobody.
exists p1 x1 x2.(n_roof(x2) & on(x1,x2) & n_firefighter(x1))
exists x2.(n_roof(x2) & -exists x1.(a_topic(x1) & on(x1,x2) & n_person(x1)))