Two people signing books.
Two authors are at a book signing.
Just because two people signing books does not mean that they are authors.
exists e1 x1 x2 x3.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_sign(e1) & n_book(x3) & (x1 = x2) & n_people(x1) & card_eq_2(x1))
exists p1 x1.(exists x2 x3.(at(x1,x2) & n_signing(x2) & of(x2,x3) & n_book(x3)) & n_author(x1) & card_eq_2(x1))