A man is sitting on the roof.
a man is on the roof
A man can only be sitting on the roof.
exists e1 x1 x2.(n_roof(x2) & on(e1,x2) & Actor(e1,x1) & v_sit(e1) & n_man(x1))
exists p1 x1 x2.(n_roof(x2) & on(x1,x2) & n_man(x1))