A zebra is nibbling on some grass.
A zebra is swallowing the grass.
A zebra can nibble grass without swallowing.
exists e1 x1 x2.(on(e1,x2) & n_grass(x2) & Actor(e1,x1) & v_nibble(e1) & n_zebra(x1))
exists e1 x1 x2.(n_grass(x2) & Theme(e1,x2) & Actor(e1,x1) & v_swallow(e1) & n_zebra(x1))