assertEquals(a, navigableMap.floorEntry(a.getKey()));
    assertEquals(a.getKey(), navigableMap.floorKey(a.getKey()));
    assertEquals(a, navigableMap.floorEntry(b.getKey()));
    assertEquals(a.getKey(), navigableMap.floorKey(b.getKey()));
    assertEquals(c, navigableMap.floorEntry(c.getKey()));
    assertEquals(c.getKey(), navigableMap.floorKey(c.getKey()));
  }

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

    assertEquals(a, navigableMap.floorEntry(a.getKey()));
    assertEquals(a.getKey(), navigableMap.floorKey(a.getKey()));
    assertEquals(a, navigableMap.floorEntry(b.getKey()));
    assertEquals(a.getKey(), navigableMap.floorKey(b.getKey()));
    assertEquals(c, navigableMap.floorEntry(c.getKey()));
    assertEquals(c.getKey(), navigableMap.floorKey(c.getKey()));
  }

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

      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));

  }

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

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

  }

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

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

  }

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

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

  @Override
  public @Nullable V get(Object key) {
    return delegate.get(checkValid(key));
  }

  @Override
  public SortedMap<K, V> headMap(K toKey) {

  }

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

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

  @Override
  public @Nullable V get(Object key) {
    return delegate.get(checkValid(key));
  }

  @Override
  public SortedMap<K, V> headMap(K toKey) {

      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));

  }

  @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 K ceilingKey(@ParametricNullness K key) {
    return Maps.keyOrNull(ceilingEntry(key));

      return standardLowerKey(key);
    }

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

    @Override
    public @Nullable K floorKey(K key) {
      return standardFloorKey(key);
    }

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

    @Override