A man wearing a headset is giving a presentation.
Nobody is wearing a headset.
Either Nobody wears a headset or a man is wearing the headset.
exists e2 e1 x1 x2 x3 x4.(Theme(e2,x4) & Actor(e2,x1) & v_give(e2) & n_presentation(x4) & Theme(e1,x3) & Actor(e1,x2) & v_wear(e1) & n_headset(x3) & (x1 = x2) & n_man(x1))
-exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_wear(e1) & n_headset(x2) & n_person(x1))