*/
  public static <K extends @Nullable Object, V extends @Nullable Object>
      SetMultimap<K, V> unmodifiableSetMultimap(SetMultimap<K, V> delegate) {
    if (delegate instanceof UnmodifiableSetMultimap || delegate instanceof ImmutableSetMultimap) {
      return delegate;
    }
    return new UnmodifiableSetMultimap<>(delegate);
  }

  /**
   * Simply returns its argument.
   *