* implement equals() (as with ListMultimap or SetMultimap, as opposed to a QueueMultimap or
   * something), our equality check is value-based.
   */
  @SuppressWarnings("UndefinedEquals")
  public void testRetainAllPropagatesToMultimap() {
    multimap().entries().retainAll(singleton(mapEntry(k0(), v0())));
    assertEquals(getSubjectGenerator().create(mapEntry(k0(), v0())), multimap());
    assertEquals(1, multimap().size());