A man speaking at a conference.
The man is at a conference.
Not all men are at a conference.
exists x1 x2 x3.(a_topic(x1) & at(x1,x3) & n_conference(x3) & n_speaking(x1) & of(x1,x2) & n_man(x2))
exists p1 x1.(n_man(x1) & exists x2.(at(x1,x2) & n_conference(x2)))