A woman is performing the backstroke in a pool.
Nobody is performing
If a woman is performing in a pool then somebody not nobody is performing.
exists e1 x1 x2 x3.(n_backstroke(x2) & in(e1,x3) & n_pool(x3) & Theme(e1,x2) & Actor(e1,x1) & v_perform(e1) & n_woman(x1))
-exists e1 x1.(Actor(e1,x1) & v_perform(e1) & n_person(x1))