return unmodifiableCollectionSubclass(output);
  }

  <E extends @Nullable Object> Collection<E> unmodifiableCollectionSubclass(
      Collection<E> collection) {
    return Collections.unmodifiableCollection(collection);
  }

  @Override
  public void clear() {
    // Clear each collection, to make previously returned collections empty.
    for (Collection<V> collection : map.values()) {

  private static class GenericClass<T> {
    private interface Base {}
  }

  private static IterableSubject makeUnmodifiable(Collection<?> actual) {
    return assertThat(Collections.<Object>unmodifiableCollection(actual));
  }

  private static class GenericClass<T> {
    private interface Base {}
  }

  private static IterableSubject makeUnmodifiable(Collection<?> actual) {
    return assertThat(Collections.<Object>unmodifiableCollection(actual));
  }