//                                                                           =========
    private static final AccessTokenDbm _instance = new AccessTokenDbm();

    private AccessTokenDbm() {
    }

    public static AccessTokenDbm getInstance() {
        return _instance;
    }

    // ===================================================================================

    List<E> tail = list.subList(1, size);
    assertTrue(copy.contains(list.get(0)));
    assertTrue(head.contains(list.get(0)));
    assertTrue(tail.contains(list.get(1)));
    // The following assumes all elements are distinct.
    assertTrue(copy.contains(list.get(size - 1)));
    assertTrue(head.contains(list.get(size - 2)));
    assertTrue(tail.contains(list.get(size - 1)));
    assertFalse(head.contains(list.get(size - 1)));

    List<E> tail = list.subList(1, size);
    assertTrue(copy.contains(list.get(0)));
    assertTrue(head.contains(list.get(0)));
    assertTrue(tail.contains(list.get(1)));
    // The following assumes all elements are distinct.
    assertTrue(copy.contains(list.get(size - 1)));
    assertTrue(head.contains(list.get(size - 2)));
    assertTrue(tail.contains(list.get(size - 1)));
    assertFalse(head.contains(list.get(size - 1)));

    //                                                                           =========
    private static final GroupDbm _instance = new GroupDbm();

    private GroupDbm() {
    }

    public static GroupDbm getInstance() {
        return _instance;
    }

    // ===================================================================================

    //                                                                           =========
    private static final LabelTypeDbm _instance = new LabelTypeDbm();

    private LabelTypeDbm() {
    }

    public static LabelTypeDbm getInstance() {
        return _instance;
    }

    // ===================================================================================

  /** Creates an empty {@code CompactLinkedHashSet} instance. */
  public static <E extends @Nullable Object> CompactLinkedHashSet<E> create() {
    return new CompactLinkedHashSet<>();
  }

  /**
   * Creates a <i>mutable</i> {@code CompactLinkedHashSet} instance containing the elements of the
   * given collection in the order returned by the collection's iterator.
   *

  /** Creates an empty {@code CompactLinkedHashMap} instance. */
  public static <K extends @Nullable Object, V extends @Nullable Object>
      CompactLinkedHashMap<K, V> create() {
    return new CompactLinkedHashMap<>();
  }

  /**
   * Creates a {@code CompactLinkedHashMap} instance, with a high enough "initial capacity" that it

    //                                                                           =========
    private static final BadWordDbm _instance = new BadWordDbm();

    private BadWordDbm() {
    }

    public static BadWordDbm getInstance() {
        return _instance;
    }

    // ===================================================================================

    //                                                                           =========
    private static final FailureUrlDbm _instance = new FailureUrlDbm();

    private FailureUrlDbm() {
    }

    public static FailureUrlDbm getInstance() {
        return _instance;
    }

    // ===================================================================================

   * actual {@code y} values of the data and the values predicted by the fit for the {@code x}
   * values (i.e. it is the square root of the mean of the squares of the vertical distances between
   * the data points and the best fit line). For this fit, this error is a fraction {@code sqrt(1 -
   * R*R)} of the population standard deviation of {@code y}, where {@code R} is the Pearson's