A boy sits on a tree stump.
A boy sitting.
sits can be replaced sitting.
exists e1 x1 x2 x3.(on(e1,x2) & n_stump(x2) & of(x2,x3) & n_tree(x3) & Actor(e1,x1) & v_sit(e1) & n_boy(x1))
exists e1 x1 x2.(a_topic(x1) & Actor(e1,x2) & v_sit(e1) & (x1 = x2) & n_boy(x1))