@Override
  public @Nullable Entry<K, V> lowerEntry(@ParametricNullness K key) {
    return delegate().lowerEntry(key);
  }

  /**
   * A sensible definition of {@link #lowerEntry} in terms of the {@code lastEntry()} of {@link
   * #headMap(Object, boolean)}. If you override {@code headMap}, you may wish to override {@code
   * lowerEntry} to forward to this implementation.
   */

  @Override
  public @Nullable Entry<K, V> lowerEntry(@ParametricNullness K key) {
    return delegate().lowerEntry(key);
  }

  /**
   * A sensible definition of {@link #lowerEntry} in terms of the {@code lastEntry()} of {@link
   * #headMap(Object, boolean)}. If you override {@code headMap}, you may wish to override {@code
   * lowerEntry} to forward to this implementation.
   */

  private Range<K> coalescedRange(Range<K> range, V value) {
    Range<K> coalescedRange = range;
    Entry<Cut<K>, RangeMapEntry<K, V>> lowerEntry =
        entriesByLowerBound.lowerEntry(range.lowerBound);
    coalescedRange = coalesce(coalescedRange, value, lowerEntry);

    Entry<Cut<K>, RangeMapEntry<K, V>> higherEntry =
        entriesByLowerBound.floorEntry(range.upperBound);

      assertTrue(Thread.holdsLock(mutex));
      return delegate().lastEntry();
    }

    @Override
    public @Nullable Entry<K, V> lowerEntry(K key) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().lowerEntry(key);
    }

    @Override
    public @Nullable K lowerKey(K key) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().lowerKey(key);
    }

      } else {
        Entry<Cut<C>, Range<C>> lowerEntry =
            rangesByLowerBound.lowerEntry(upperBoundWindow.lowerEndpoint());
        if (lowerEntry == null) {
          backingItr = rangesByLowerBound.values().iterator();
        } else if (upperBoundWindow.lowerBound.isLessThan(lowerEntry.getValue().upperBound)) {
          backingItr = rangesByLowerBound.tailMap(lowerEntry.getKey(), true).values().iterator();
        } else {

      return (NavigableMap<K, Collection<V>>) super.sortedMap();
    }

    @Override
    public @Nullable Entry<K, Collection<V>> lowerEntry(@ParametricNullness K key) {
      Entry<K, Collection<V>> entry = sortedMap().lowerEntry(key);
      return (entry == null) ? null : wrapEntry(entry);
    }

    @Override
    public @Nullable K lowerKey(@ParametricNullness K key) {

      return (NavigableMap<K, Collection<V>>) super.sortedMap();
    }

    @Override
    public @Nullable Entry<K, Collection<V>> lowerEntry(@ParametricNullness K key) {
      Entry<K, Collection<V>> entry = sortedMap().lowerEntry(key);
      return (entry == null) ? null : wrapEntry(entry);
    }

    @Override
    public @Nullable K lowerKey(@ParametricNullness K key) {

  }

  <K, V> void testNavigationAgainstExpected(
      NavigableMap<K, V> expected, NavigableMap<K, V> navigableMap, Iterable<K> keysToTest) {
    for (K key : keysToTest) {
      assertEquals(expected.lowerEntry(key), navigableMap.lowerEntry(key));
      assertEquals(expected.floorEntry(key), navigableMap.floorEntry(key));
      assertEquals(expected.ceilingEntry(key), navigableMap.ceilingEntry(key));