Multiset<String> mod = HashMultiset.create();
    Multiset<String> unmod = unmodifiableMultiset(mod);
    assertNotSame(mod, unmod);
    assertSame(unmod, unmodifiableMultiset(unmod));
    ImmutableMultiset<String> immutable = ImmutableMultiset.of("a", "a", "b", "a");
    assertSame(immutable, unmodifiableMultiset(immutable));
    assertSame(immutable, unmodifiableMultiset((Multiset<String>) immutable));
  }