docs: @Nonnull
    public static EChange safeInvalidateSession (@Nullable final HttpSession aSession)
    {
      if (aSession != null)
      {
        try
        {
          if (LOGGER.isDebugEnabled ())
            LOGGER.debug ("Invalidating session " + aSession.getId ());
          aSession.invalidate ();
          return EChange.CHANGED;
        }
        catch (final IllegalStateException ex)
        {
          // session already invalidated
        }
      }
      return EChange.UNCHANGED;
    }