A man sitting upon a roof is using a tool.
Nobody is using a tool.
If nobody is using a tool, then it cannot also be that a man is using a tool
exists e2 e1 x1 x2 x3 x4.(Theme(e2,x4) & Actor(e2,x1) & v_use(e2) & n_tool(x4) & upon(e1,x3) & n_roof(x3) & Actor(e1,x2) & v_sit(e1) & (x1 = x2) & n_man(x1))
-exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_use(e1) & n_tool(x2) & n_person(x1))