assertEquals(null, navigableMap.lowerEntry(a.getKey()));
    assertEquals(null, navigableMap.lowerKey(a.getKey()));
    assertEquals(a, navigableMap.lowerEntry(b.getKey()));
    assertEquals(a.getKey(), navigableMap.lowerKey(b.getKey()));
    assertEquals(a, navigableMap.lowerEntry(c.getKey()));
    assertEquals(a.getKey(), navigableMap.lowerKey(c.getKey()));
  }

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

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

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

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

  }

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

  @Override
  public @Nullable K lowerKey(K key) {
    return delegate.lowerKey(checkValid(key));
  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return delegate.navigableKeySet();
  }

  @Override

  }

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

  @Override
  public @Nullable K lowerKey(K key) {
    return delegate.lowerKey(checkValid(key));
  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return delegate.navigableKeySet();
  }

  @Override

  }

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

  @Override
  @CheckForNull
  public K lowerKey(@ParametricNullness K key) {
    return Maps.keyOrNull(lowerEntry(key));
  }

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

  }

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

  @Override
  @CheckForNull
  public K lowerKey(@ParametricNullness K key) {
    return Maps.keyOrNull(lowerEntry(key));
  }

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

      return standardSubMap(fromKey, toKey);
    }

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

    @Override
    public @Nullable K lowerKey(K key) {
      return standardLowerKey(key);
    }

    @Override
    public @Nullable Entry<K, V> floorEntry(K key) {
      return standardFloorEntry(key);
    }

    @Override

    public Entry<K, V2> lowerEntry(@ParametricNullness K key) {
      return transformEntry(fromMap().lowerEntry(key));
    }

    @Override
    @CheckForNull
    public K lowerKey(@ParametricNullness K key) {
      return fromMap().lowerKey(key);
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return fromMap().navigableKeySet();
    }

    @Override
    @CheckForNull

    public Entry<K, V2> lowerEntry(@ParametricNullness K key) {
      return transformEntry(fromMap().lowerEntry(key));
    }

    @Override
    @CheckForNull
    public K lowerKey(@ParametricNullness K key) {
      return fromMap().lowerKey(key);
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return fromMap().navigableKeySet();
    }

    @Override
    @CheckForNull

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

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

    @Override
    public Set<K> keySet() {
      return navigableKeySet();
    }