public void testPermutationSetEmpty() {
    Collection<List<Integer>> permutationSet =
        Collections2.permutations(Collections.<Integer>emptyList());

    assertEquals(1, permutationSet.size());
    assertTrue(permutationSet.contains(Collections.<Integer>emptyList()));

    Iterator<List<Integer>> permutations = permutationSet.iterator();
    assertNextPermutation(Collections.<Integer>emptyList(), permutations);

      K key = entry.getKey();
      V value = entry.getValue();
      Collection<V> collection = multimap().get(key);
      assertNotNull(collection);
      Collection<V> expectedCollection = copyToList(collection);

      multimap().remove(key, value);
      expectedCollection.remove(value);

      assertEqualIgnoringOrder(expectedCollection, collection);
      assertEquals(!expectedCollection.isEmpty(), multimap().containsKey(key));

      K key = entry.getKey();
      V value = entry.getValue();
      Collection<V> collection = multimap().get(key);
      assertNotNull(collection);
      Collection<V> expectedCollection = copyToList(collection);

      multimap().remove(key, value);
      expectedCollection.remove(value);

      assertEqualIgnoringOrder(expectedCollection, collection);
      assertEquals(!expectedCollection.isEmpty(), multimap().containsKey(key));

     *
     * @param parser The XML parser
     * @return Converted Collection instance
     */
    private Collection<Object> parseCollection(final MXParser parser, final TypeLiteral<?> toType) throws Exception {
        @SuppressWarnings("unchecked")
        final Collection<Object> collection = newImplementation(parser, ArrayList.class);

    @Override
    @SuppressWarnings({"unchecked", "rawtypes"}) // needed for arrays
    public Entry<String, Collection<Integer>>[] createArray(int length) {
      return new Entry[length];
    }

    @Override
    public Iterable<Entry<String, Collection<Integer>>> order(
        List<Entry<String, Collection<Integer>>> insertionOrder) {
      return insertionOrder;
    }

    @Override

    new ForwardingWrapperTester()
        .testForwarding(
            Collection.class,
            new Function<Collection, Collection>() {
              @Override
              public Collection apply(Collection delegate) {
                return wrap(delegate);
              }
            });
  }

  private static <T> Collection<T> wrap(Collection<T> delegate) {
    return new ForwardingCollection<T>() {
      @Override

  public void testEntrySetEquals() {
    resetContainer(mapEntry(k0(), v0()), mapEntry(k1(), v0()), mapEntry(k0(), v3()));
    Set<Entry<K, Collection<V>>> expected = new HashSet<>();
    expected.add(mapEntry(k0(), (Collection<V>) newHashSet(v0(), v3())));
    expected.add(mapEntry(k1(), (Collection<V>) newHashSet(v0())));
    new EqualsTester().addEqualityGroup(expected, multimap().asMap().entrySet()).testEquals();
  }

  public void testRemove_nullPresent() {
    collection = getSubjectGenerator().create(createArrayWithNullElement());

    int initialSize = collection.size();
    assertTrue("remove(null) should return true", collection.remove(null));
    assertEquals(
        "remove(present) should decrease a collection's size by one.",
        initialSize - 1,
        collection.size());
    expectMissing((E) null);
  }

    public CoreExtensionEntry(
            ClassRealm realm,
            Collection<String> artifacts,
            Collection<String> packages,
            String key,
            XmlNode configuration) {
        this.realm = realm;
        this.artifacts = Collections.unmodifiableSet(new HashSet<>(artifacts));
        this.packages = Collections.unmodifiableSet(new HashSet<>(packages));
        this.key = key;

        () -> {
          Iterator<E> iterator = collection.iterator();
          assertTrue(collection.add(e3()));
          iterator.next();
        });
  }

  /**
   * Returns the {@link Method} instance for {@link #testAdd_nullSupported()} so that tests of
   * {@link java.util.Collections#checkedCollection(java.util.Collection, Class)} can suppress it