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

theorem Th12:
  born x = born -x
proof
  x in Day born x & -x in Day born -x by SURREAL0:def 18;
  then -x in Day born x & - - x in Day born -x & - -x = x by Lm1;
  then born -x c= born x & born x c= born -x by SURREAL0:def 18;
  hence thesis by XBOOLE_0:def 10;
end;
