A girl playing her guitar in her bedroom.
A boy is sitting at his piano.
There can either be a girl or boy performing an action, not both. The person cannot be playing a different instrument, guitar or piano, at the same time. Although the person playing the guitar could be sitting, they are actively playing, which is not necessarily implied with the statement of the person merely sitting at the piano.
exists e1 x1 x2 x3 x4.(a_topic(x1) & in(e1,x4) & n_bedroom(x4) & of(x4,x1) & n_female(x1) & n_female(x1) & Theme(e1,x3) & Actor(e1,x2) & v_play(e1) & n_guitar(x3) & of(x3,x1) & (x1 = x2) & n_girl(x1))
exists e1 x1 x2.(at(e1,x2) & n_piano(x2) & of(x2,x1) & n_male(x1) & Actor(e1,x1) & v_sit(e1) & n_boy(x1))