An informally dressed orchestra is rehearsing in an empty auditorium.
A group of people practicing for a concert
A group of people rehearsing do not have to be doing so for a concert.
exists e1 e2 s1 x1 x2.(in(e2,x2) & n_auditorium(x2) & a_empty(s1) & Theme(s1,x2) & Actor(e2,x1) & v_rehearse(e2) & n_orchestra(x1) & Theme(e1,x1) & v_dress(e1) & r_informally(x1))
exists e1 x1 x2 x3 x4.(a_topic(x1) & of(x1,x2) & for(e1,x4) & n_concert(x4) & Actor(e1,x3) & v_practice(e1) & (x2 = x3) & n_people(x2) & n_group(x1))