assertEquals(c, navigableMap.higherEntry(a.getKey()));
    assertEquals(c.getKey(), navigableMap.higherKey(a.getKey()));
    assertEquals(c, navigableMap.higherEntry(b.getKey()));
    assertEquals(c.getKey(), navigableMap.higherKey(b.getKey()));
    assertEquals(null, navigableMap.higherEntry(c.getKey()));
    assertEquals(null, navigableMap.higherKey(c.getKey()));
  }

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

      return standardCeilingKey(key);
    }

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

    @Override
    public @Nullable K higherKey(K key) {
      return standardHigherKey(key);
    }

    @Override
    public @Nullable Entry<K, V> firstEntry() {
      return standardFirstEntry();
    }

    /*

      Entry<K, Collection<V>> entry = sortedMap().higherEntry(key);
      return (entry == null) ? null : wrapEntry(entry);
    }

    @Override
    public @Nullable K higherKey(@ParametricNullness K key) {
      return sortedMap().higherKey(key);
    }

    @Override
    public @Nullable Entry<K, Collection<V>> firstEntry() {
      Entry<K, Collection<V>> entry = sortedMap().firstEntry();

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

    @Override
    public @Nullable K higherKey(@ParametricNullness K key) {
      return fromMap().higherKey(key);
    }

    @Override
    public @Nullable Entry<K, V2> lastEntry() {
      return transformEntry(fromMap().lastEntry());
    }

    @Override

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

    @Override
    public @Nullable K higherKey(@ParametricNullness K key) {
      return fromMap().higherKey(key);
    }

    @Override
    public @Nullable Entry<K, V2> lastEntry() {
      return transformEntry(fromMap().lastEntry());
    }

    @Override

    assertEquals(ImmutableSortedMap.of("three", 5), map.subMap("one", false, "tr", true));

    assertEquals("three", map.higherKey("one"));
    assertEquals("three", map.higherKey("r"));
    assertEquals("three", map.ceilingKey("r"));
    assertEquals("one", map.ceilingKey("one"));
    assertEquals(mapEntry("three", 5), map.higherEntry("one"));

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

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

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

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

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

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

    assertEquals(ImmutableSortedMap.of("three", 5), map.subMap("one", false, "tr", true));

    assertEquals("three", map.higherKey("one"));
    assertEquals("three", map.higherKey("r"));
    assertEquals("three", map.ceilingKey("r"));
    assertEquals("one", map.ceilingKey("one"));
    assertEquals(mapEntry("three", 5), map.higherEntry("one"));

  }

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

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

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

  @Override