assertThrows(IllegalArgumentException.class, () -> filtered.put("cow", 7));
    assertEquals(ImmutableMap.of("cat", 3, "horse", 5, "chicken", 7), filtered);
  }

  public void testFilteredEntriesIllegalPutAll() {
    Map<String, Integer> unfiltered = createUnfiltered();
    unfiltered.put("cat", 3);
    unfiltered.put("dog", 2);
    unfiltered.put("horse", 5);