BiMap<String, Integer> bimap = ImmutableBiMap.copyOf(ImmutableMap.of("one", 1, "two", 2));
    assertThrows(UnsupportedOperationException.class, () -> bimap.forcePut("three", 3));
  }

  public void testKeySet() {
    ImmutableBiMap<String, Integer> bimap =
        ImmutableBiMap.copyOf(ImmutableMap.of("one", 1, "two", 2, "three", 3, "four", 4));
    Set<String> keys = bimap.keySet();