reserve X for set;
reserve UN for Universe;

theorem Th54:
  for UN being non trivial Universe holds ExtREAL is Element of UN
  proof
    let UN being non trivial Universe;
    reconsider u = REAL as Element of UN by Th53;
    0 is Element of UN by Th13;
    then [0,u] is Element of UN by CLASSES2:3;
    then {u,[0,u]} is Element of UN by CLASSES2:2;
    hence thesis by CLASSES2:60;
  end;
