assertTrue(Thread.holdsLock(mutex));
      return delegate().floorEntry(key);
    }

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

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      assertTrue(Thread.holdsLock(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();

        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();