A female singer and male guitarist on stage.
Nobody is on stage
Singer and guitarist can not be on stage is nobody is on stage.
exists s1 s2 x1 x2 x3.(a_topic(x1) & on(x1,x3) & n_stage(x3) & n_guitarist(x1) & a_male(s2) & Theme(s2,x1) & of(x1,x2) & n_singer(x2) & a_female(s1) & Theme(s1,x1))
-exists p1 x1.(exists x2.(on(x1,x2) & n_stage(x2)) & n_person(x1))