A man performing dangerous job duties.
Nobody is performing.
A man is performing means someone is performing, not nobody.
exists e1 s1 x1 x2 x3 x4.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_perform(e1) & n_duty(x3) & of(x3,x4) & n_job(x4) & a_dangerous(s1) & Theme(s1,x3) & (x1 = x2) & n_man(x1))
-exists e1 x1.(Actor(e1,x1) & v_perform(e1) & n_person(x1))