A zebra is nibbling on some grass.
A zebra is not nibbling on grass.
Nibbling and not nibbling are different.
exists e1 x1 x2.(on(e1,x2) & n_grass(x2) & Actor(e1,x1) & v_nibble(e1) & n_zebra(x1))
exists x1.(-exists e1 x2.(on(e1,x2) & n_grass(x2) & Actor(e1,x1) & v_nibble(e1)) & n_zebra(x1))