*
   * <p><b>Warning:</b> If the function rejects {@code null}, caution is required to make sure the
   * set does not contain {@code null}, because the view cannot stop {@code null} from being added
   * to the set.
   *
   * <p><b>Warning:</b> This method assumes that for any instance {@code k} of key type {@code K},
   * {@code k.equals(k2)} implies that {@code k2} is also of type {@code K}. Using a key type for