public Entry<K, V2> pollFirstEntry() {
      return transformEntry(fromMap().pollFirstEntry());
    }

    @Override
    @CheckForNull
    public Entry<K, V2> pollLastEntry() {
      return transformEntry(fromMap().pollLastEntry());
    }

    @Override
    public NavigableMap<K, V2> subMap(
        @ParametricNullness K fromKey,
        boolean fromInclusive,

        return nullableSynchronizedEntry(delegate().pollFirstEntry(), mutex);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> pollLastEntry() {
      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().pollLastEntry(), mutex);
      }
    }

    @Override
    public NavigableMap<K, V> subMap(

        return nullableSynchronizedEntry(delegate().pollFirstEntry(), mutex);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> pollLastEntry() {
      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().pollLastEntry(), mutex);
      }
    }

    @Override
    public NavigableMap<K, V> subMap(

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  @CheckForNull
  public final Entry<E> pollLastEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public abstract ImmutableSortedMultiset<E> headMultiset(E upperBound, BoundType boundType);

  @Override

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  @CheckForNull
  public final Entry<K, V> pollLastEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public ImmutableSortedMap<K, V> descendingMap() {
    // TODO(kevinb): The descendingMap is never actually cached at all. Either:

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  @CheckForNull
  public final Entry<K, V> pollLastEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public ImmutableSortedMap<K, V> descendingMap() {
    // TODO(kevinb): The descendingMap is never actually cached at all. Either: