reserve A,B,C for Ordinal,
        o for object,
        x,y,z,t,r,l for Surreal,
        X,Y for set;

theorem
  for X be surreal-membered set holds
    card X = card --X
proof
  let X be surreal-membered set;
  -- -- X = X by Th15;
  then card (--X) c= card X c= card (--X) by Th16;
  hence thesis by XBOOLE_0:def 10;
end;
