assertThrows(
        UnsupportedOperationException.class, () -> map.headMap("r").tailMap("m").keySet().add("a"));
  }

  public void testAsMapSortedEmpty() {
    SortedSet<String> strings = new NonNavigableSortedSet();
    SortedMap<String, Integer> map = Maps.asMap(strings, LENGTH_FUNCTION);
    assertThat(map.entrySet()).isEmpty();
    assertTrue(map.isEmpty());