Two people signing books.
Two people buying used books.
The people must be either signing or buying the books.
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 e2 e1 x1 x2 x3.(a_topic(x1) & Theme(e2,x3) & Actor(e2,x2) & v_buy(e2) & n_book(x3) & Theme(e1,x3) & v_use(e1) & (x1 = x2) & n_people(x1) & card_eq_2(x1))