int key = 1;
      for (UnhashableObject value : elements) {
        builder.put(key++, value);
      }
      return builder.buildOrThrow().values();
    }
  }

  public static class ImmutableMapKeyListGenerator extends TestStringListGenerator {
    @Override
    public List<String> create(String[] elements) {
      ImmutableMap.Builder<String, Integer> builder = ImmutableMap.builder();