A group of people are at a rock concert.
People are dancing at a rock concert.
Just because people are at a rock concert it doesn't mean they're dancing.
exists p1 x1 x2.(exists x3 x4.(at(x1,x3) & n_concert(x3) & of(x3,x4) & n_rock(x4)) & of(x1,x2) & n_people(x2) & n_group(x1))
exists e1 x1 x2 x3.(at(e1,x2) & n_concert(x2) & of(x2,x3) & n_rock(x3) & Actor(e1,x1) & v_dance(e1) & n_people(x1))