theorem
  |{ p, q, p <X> q }| = |. p <X> q .|^2
  proof
    |{ p, q, p <X> q }| = |(q,q)| * |(p,p)| - |(q,p)| * |(p,q)| by Th45
                       .= |(p <X> q,p <X> q)| by Th46;
    hence thesis by EUCLID_2:4;
  end;
