A family playing on a sand dune.
a family plays in front of another family
A family playing on a sand dune doesn't imply that they are in front of another family.
exists e1 x1 x2 x3 x4.(a_topic(x1) & on(e1,x3) & n_dune(x3) & of(x3,x4) & n_sand(x4) & Actor(e1,x2) & v_play(e1) & (x1 = x2) & n_family(x1))
exists e1 x1 x2 x3.(in(e1,x2) & of(x2,x3) & n_family(x3) & -(x3 = x1) & n_family(x1) & n_family(x1) & n_front(x2) & Actor(e1,x1) & v_play(e1))