reserve fi,psi for Ordinal-Sequence,
  A,A1,B,C,D for Ordinal,
  X,Y for set,
  x,y for object;

theorem Th24:
  A c= A+^B & B c= A+^B
proof
A1: {}+^B c= A+^B by ORDINAL2:34,XBOOLE_1:2;
  A+^{} c= A+^B by ORDINAL2:33,XBOOLE_1:2;
  hence thesis by A1,ORDINAL2:27,30;
end;
