guava/src/com/google/common/collect/Multisets.java
@Deprecated
  public static <E> Multiset<E> unmodifiableMultiset(ImmutableMultiset<E> multiset) {
    return checkNotNull(multiset);
  }
@Deprecated
  public static <E> Multiset<E> unmodifiableMultiset(ImmutableMultiset<E> multiset) {
    return checkNotNull(multiset);
  }
Simply returns its argument.

@deprecated no need to use this
@since 10.0