A welder person in the middle of a weld.
Nobody is welding
There can either be a welder or nobody welding.
exists x1 x2 x3 x4.(n_middle(x3) & a_topic(x1) & in(x1,x3) & of(x3,x4) & n_weld(x4) & n_person(x1) & of(x1,x2) & n_welder(x2))
-exists e1 x1.(Actor(e1,x1) & v_weld(e1) & n_person(x1))