theorem :: Proposition 7 b2)
  (delta_2 R).(X,Y) = (delta_2 R).(Y,X)
  proof
    (delta_2 R).(X,Y) = (CMap kappa_2 R).(X,Y) + (CMap kappa_2 R).(Y,X)
      by Delta2;
    hence thesis by Delta2;
  end;
