assertEquals(a, navigableMap.floorEntry(a.getKey()));
    assertEquals(a.getKey(), navigableMap.floorKey(a.getKey()));
    assertEquals(a, navigableMap.floorEntry(b.getKey()));
    assertEquals(a.getKey(), navigableMap.floorKey(b.getKey()));
    assertEquals(c, navigableMap.floorEntry(c.getKey()));
    assertEquals(c.getKey(), navigableMap.floorKey(c.getKey()));
  }

  @CollectionSize.Require(SEVERAL)
  public void testCeiling() {

        if (subRange.isEmpty()) {
          return emptyIterator();
        }
        Cut<K> cutToStart =
            MoreObjects.firstNonNull(
                entriesByLowerBound.floorKey(subRange.lowerBound), subRange.lowerBound);
        final Iterator<RangeMapEntry<K, V>> backingItr =
            entriesByLowerBound.tailMap(cutToStart, true).values().iterator();

        return nullableSynchronizedEntry(delegate().floorEntry(key), mutex);
      }
    }

    @Override
    @CheckForNull
    public K floorKey(K key) {
      synchronized (mutex) {
        return delegate().floorKey(key);
      }
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {

        return nullableSynchronizedEntry(delegate().floorEntry(key), mutex);
      }
    }

    @Override
    @CheckForNull
    public K floorKey(K key) {
      synchronized (mutex) {
        return delegate().floorKey(key);
      }
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {

        if (subRange.isEmpty()) {
          return emptyIterator();
        }
        Cut<K> cutToStart =
            MoreObjects.firstNonNull(
                entriesByLowerBound.floorKey(subRange.lowerBound), subRange.lowerBound);
        final Iterator<RangeMapEntry<K, V>> backingItr =
            entriesByLowerBound.tailMap(cutToStart, true).values().iterator();

  }

  @Override
  @CheckForNull
  public Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override
  @CheckForNull
  public K floorKey(K key) {
    return keyOrNull(floorEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> ceilingEntry(K key) {
    return tailMap(key, true).firstEntry();
  }

  @Override

  }

  @Override
  @CheckForNull
  public Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override
  @CheckForNull
  public K floorKey(K key) {
    return keyOrNull(floorEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> ceilingEntry(K key) {
    return tailMap(key, true).firstEntry();
  }

  @Override