A singer of a band on stage.
A gutarist of a band onstage.
There can be a singer or a guitarist.
exists x1 x2 x3.(a_topic(x1) & of(x1,x2) & on(x2,x3) & n_stage(x3) & n_band(x2) & n_singer(x1))
exists x1 x2 x3.(a_topic(x1) & of(x1,x2) & n_onstage(x2) & of(x2,x3) & n_band(x3) & n_gutarist(x1))