public abstract Set<String> getStringSet();
@Test
  public void getStringSet() {
    // Test that getStringSet gives an immutable set
    HashSet<String> initialSet = new HashSet<>(Arrays.asList("ab", "cd"));
    Set<String> stringSetResultSet = StringSetResult.create(initialSet).getStringSet();
    assertEquals(initialSet, stringSetResultSet);
    assertThrows(UnsupportedOperationException.class, () -> stringSetResultSet.add("should-fail"));
  }