public NavigableMap<K, V2> descendingMap() {
      return transformEntries(fromMap().descendingMap(), transformer);
    }

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

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

    {
      Entry<Integer, String> entry = unmod.higherEntry(2);
      assertThrows(UnsupportedOperationException.class, () -> entry.setValue("four"));
    }
    {
      Entry<Integer, String> entry = unmod.firstEntry();
      assertThrows(UnsupportedOperationException.class, () -> entry.setValue("four"));
    }
    {
      Entry<Integer, String> entry = unmod.lastEntry();

    {
      Entry<Integer, String> entry = unmod.higherEntry(2);
      assertThrows(UnsupportedOperationException.class, () -> entry.setValue("four"));
    }
    {
      Entry<Integer, String> entry = unmod.firstEntry();
      assertThrows(UnsupportedOperationException.class, () -> entry.setValue("four"));
    }
    {
      Entry<Integer, String> entry = unmod.lastEntry();