public static StringSetResult empty() {
    return EmptyStringSetResult.INSTANCE;
  }
@Test
  public void empty() {
    // Test empty returns an immutable set
    StringSetResult empptyStringSetResult = StringSetResult.empty();
    assertTrue(empptyStringSetResult.getStringSet().isEmpty());
    assertThrows(
        UnsupportedOperationException.class,
        () -> empptyStringSetResult.getStringSet().add("should-fail"));
  }