@CanIgnoreReturnValue
  public final Ordered containsExactly() {
    return containsExactlyEntriesIn(ImmutableMap.of());
  }
@Test
  public void containsExactlyEmpty_fails() {
    ImmutableMap<String, Integer> actual = ImmutableMap.of("jan", 1);

    expectFailureWhenTestingThat(actual).containsExactly();
    assertFailureKeys("expected to be empty", "but was");
  }