public static <E extends Enum<E>> FlagSet<E> createFlagSet(
      final Class<E> enumClass,
      final String prefix,
      final EnumSet<E> flags) {
    return new FlagSet<>(enumClass, prefix, flags);
  }
@Test
  public void testCreateNullEnumClass() throws Throwable {
    intercept(NullPointerException.class, () ->
        createFlagSet(null, KEYDOT, SimpleEnum.a));
  }