public void clear() {
      ReferenceEntry<K, V> e = head.getNextInWriteQueue();
      while (e != head) {
        ReferenceEntry<K, V> next = e.getNextInWriteQueue();
        nullifyWriteOrder(e);
        e = next;
      }

      head.setNextInWriteQueue(head);
      head.setPreviousInWriteQueue(head);
    }

    @Override
    public Iterator<ReferenceEntry<K, V>> iterator() {

    ticker.advance(1);
    segment.recordRead(entry, ticker.read());
    segment.expireEntries(ticker.read());
    assertTrue(map.containsKey(key));
    assertSame(entry, segment.accessQueue.peek());
    assertEquals(1, segment.accessQueue.size());

    ticker.advance(1);
    segment.recordRead(entry, ticker.read());
    segment.expireEntries(ticker.read());
    assertTrue(map.containsKey(key));

    ticker.advance(1);
    segment.recordRead(entry, ticker.read());
    segment.expireEntries(ticker.read());
    assertTrue(map.containsKey(key));
    assertSame(entry, segment.accessQueue.peek());
    assertEquals(1, segment.accessQueue.size());

    ticker.advance(1);
    segment.recordRead(entry, ticker.read());
    segment.expireEntries(ticker.read());
    assertTrue(map.containsKey(key));

    public void clear() {
      ReferenceEntry<K, V> e = head.getNextInWriteQueue();
      while (e != head) {
        ReferenceEntry<K, V> next = e.getNextInWriteQueue();
        nullifyWriteOrder(e);
        e = next;
      }

      head.setNextInWriteQueue(head);
      head.setPreviousInWriteQueue(head);
    }

    @Override
    public Iterator<ReferenceEntry<K, V>> iterator() {

   * Example usage:
   *
   * <pre>{@code
   * Color red = new Color("red", 255, 0, 0);
   * ...
   * ImmutableSet<Color> allColors = ImmutableSet.of(red, green, blue);
   *
   * ImmutableMap<String, Color> colorForName =
   *     uniqueIndex(allColors, c -> c.toString());
   * assertThat(colorForName).containsEntry("red", red);
   * }</pre>
   *