code: public static <K, V> SortedSetMultimap<K, V> unmodifiableSortedSetMultimap(
        SortedSetMultimap<K, V> delegate) {
      if (delegate instanceof UnmodifiableSortedSetMultimap) {
        return delegate;
      }
      return new UnmodifiableSortedSetMultimap<>(delegate);
    }
docs: Returns an unmodifiable view of the specified {@code SortedSetMultimap}. Query operations on
  the returned multimap "read through" to the specified multimap, and attempts to modify the
  returned multimap, either directly or through the multimap's views, result in an {@code
  UnsupportedOperationException}.
  
  <p>The returned multimap will be serializable if the specified multimap is serializable.
  
  @param delegate the multimap for which an unmodifiable view is to be returned
  @return an unmodifiable view of the specified multimap