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

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

    @Override
    @CheckForNull
    public Map.Entry<K, V> lastEntry() {
      synchronized (mutex) {

  }

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

  @Override
  @CheckForNull
  public K higherKey(K key) {
    return keyOrNull(higherEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> firstEntry() {
    return isEmpty() ? null : entrySet().asList().get(0);
  }

  }

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

  @Override
  @CheckForNull
  public K higherKey(K key) {
    return keyOrNull(higherEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> firstEntry() {
    return isEmpty() ? null : entrySet().asList().get(0);
  }