code: public static <K, V> Multimap<K, V> unmodifiableMultimap(Multimap<K, V> multimap) {
          if (multimap instanceof UnmodifiableMultimap) {
              return multimap;
          }
          return new UnmodifiableMultimap<K, V>(multimap);
      }
docs: Note that {@link Multimap#get(Object)} always returns unmodifiable collections. Moreover, it does not trigger initialization of a new value collection
  (i.e. when no collection of values for a given key exists).
  
  @param multimap
  @return an unmodifiable view of the given multimap