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() {

    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();

    return headMap(key, false).lastEntry();
  }

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

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

    return headMap(key, false).lastEntry();
  }

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

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

  }

  @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

      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
  @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