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

theorem Th30:
  for X,Y be set holds X++Y = Y++X
proof
  let X,Y be  set;
  X++Y c= Y++X  c= X ++ Y by Lm4;
  hence thesis by XBOOLE_0:def 10;
end;
