public final void isEmpty() {
    if (!checkNotNull(actual).isEmpty()) {
      failWithActual(simpleFact("expected to be empty"));
    }
  }
@Test
  public void multimapIsEmptyWithFailure() {
    ImmutableMultimap<Integer, Integer> multimap = ImmutableMultimap.of(1, 5);
    expectFailureWhenTestingThat(multimap).isEmpty();
    assertFailureKeys("expected to be empty", "but was");
  }