A drummer sits behind a guitar player while they perform on stage.
A drummer and guitar playing are the opening act on stage.
You can not infer they are the opening act.
exists e1 p1 x1 x2 x3.(exists e2 x4.(on(e2,x4) & n_stage(x4) & Actor(e2,x1) & v_perform(e2)) & n_thing(x1) & while(e1,p1) & behind(e1,x2) & n_player(x2) & of(x2,x3) & n_guitar(x3) & Actor(e1,x1) & v_sit(e1) & n_drummer(x1))
exists e1 p1 x1 x2 x3 x4 x5.(n_act(x5) & Actor(e1,x5) & v_open(e1) & exists x6.((x1 = x5) & on(x5,x6) & n_stage(x6)) & n_playing(x3) & of(x3,x4) & n_guitar(x4) & n_drummer(x2) & subset_of(x3,x1) & subset_of(x2,x1))