A zebra is nibbling on some grass.
The zebra is being stalked by a lion.
Just because a zebra is nibbling on grass doesn't mean that it is being stalked by a lion.
exists e1 x1 x2.(on(e1,x2) & n_grass(x2) & Actor(e1,x1) & v_nibble(e1) & n_zebra(x1))
exists e1 x1 x2.(n_zebra(x1) & Actor(e1,x2) & n_lion(x2) & Theme(e1,x1) & v_stalk(e1))