A man is working on steel.
The man is working on his garden.
The same subject cannot be working on steel and his garden.
exists e1 x1 x2.(on(e1,x2) & n_steel(x2) & Actor(e1,x1) & v_work(e1) & n_man(x1))
exists e1 x1 x2.(n_garden(x2) & of(x2,x1) & n_male(x1) & n_man(x1) & on(e1,x2) & Actor(e1,x1) & v_work(e1))