A girl walking on rocks.
A girl walks across the sand.
Rocks are different than sand.
exists e1 x1 x2 x3.(a_topic(x1) & on(e1,x3) & n_rock(x3) & Actor(e1,x2) & v_walk(e1) & (x1 = x2) & n_girl(x1))
exists e1 x1 x2.(n_sand(x2) & across(e1,x2) & Actor(e1,x1) & v_walk(e1) & n_girl(x1))