SetMultimap<String, Integer> unmod = Multimaps.unmodifiableSetMultimap(mod);
    assertNotSame(mod, unmod);
    assertSame(unmod, Multimaps.unmodifiableSetMultimap(unmod));
    ImmutableSetMultimap<String, Integer> immutable =
        ImmutableSetMultimap.of("a", 1, "b", 2, "a", 3);
    assertSame(immutable, Multimaps.unmodifiableSetMultimap(immutable));
    assertSame(

   */
  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.
   *