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

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

    @Override
    public NavigableSet<K> descendingKeySet() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().descendingKeySet();

  }

  @Override
  @CheckForNull
  public K ceilingKey(@ParametricNullness K key) {
    return delegate().ceilingKey(key);
  }

  /**
   * A sensible definition of {@link #ceilingKey} in terms of {@code ceilingEntry}. If you override
   * {@code ceilingEntry}, you may wish to override {@code ceilingKey} to forward to this
   * implementation.
   */
  @CheckForNull

  }

  @Override
  @CheckForNull
  public K ceilingKey(@ParametricNullness K key) {
    return delegate().ceilingKey(key);
  }

  /**
   * A sensible definition of {@link #ceilingKey} in terms of {@code ceilingEntry}. If you override
   * {@code ceilingEntry}, you may wish to override {@code ceilingKey} to forward to this
   * implementation.
   */
  @CheckForNull

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

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

    @Override
    public NavigableSet<K> descendingKeySet() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().descendingKeySet();

      return (entry == null) ? null : wrapEntry(entry);
    }

    @Override
    @CheckForNull
    public K ceilingKey(@ParametricNullness K key) {
      return sortedMap().ceilingKey(key);
    }

    @Override
    @CheckForNull
    public Entry<K, Collection<V>> higherEntry(@ParametricNullness K key) {

      return (entry == null) ? null : wrapEntry(entry);
    }

    @Override
    @CheckForNull
    public K ceilingKey(@ParametricNullness K key) {
      return sortedMap().ceilingKey(key);
    }

    @Override
    @CheckForNull
    public Entry<K, Collection<V>> higherEntry(@ParametricNullness K key) {

      if (lowerEntry != null
          && lowerEntry.getValue().getUpperBound().compareTo(subRange.lowerBound) > 0) {
        lowerBound = subRange.lowerBound;
      } else {
        lowerBound = entriesByLowerBound.ceilingKey(subRange.lowerBound);
        if (lowerBound == null || lowerBound.compareTo(subRange.upperBound) >= 0) {
          throw new NoSuchElementException();
        }
      }

      Cut<K> upperBound;