People are observing billboards.
The people are sleeping.
People observing billboards cannot be the ones sleeping.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_observe(e1) & n_billboard(x2) & n_people(x1))
exists e1 x1.(n_people(x1) & Actor(e1,x1) & v_sleep(e1))