public ImmutableSet<E> elementSet() {
    ImmutableSet<E> result = elementSet;
    return (result == null) ? elementSet = new ElementSet() : result;
  }

  @WeakOuter
  private final class ElementSet extends IndexedImmutableSet<E> {

    @Override
    E get(int index) {
      return contents.getKey(index);
    }

    @Override
    public boolean contains(@CheckForNull Object object) {