A man is welding something on a cinder block.
Nobody is welding
If someone is welding, then it is not possible for nobody to be welding.
exists e1 x1 x2 x3 x4.(on(e1,x3) & n_block(x3) & of(x3,x4) & n_cinder(x4) & Theme(e1,x2) & Actor(e1,x1) & v_weld(e1) & n_thing(x2) & n_man(x1))
-exists e1 x1.(Actor(e1,x1) & v_weld(e1) & n_person(x1))