code: public Multimap<String, String> getOrCreate(String index) {
          Multimap<String, String> mmap = storeMap.get(index);
          if (mmap == null) {
              SetMultimap<String, String> multimap =
                      Multimaps.newSetMultimap(new HashMap<String, Collection<String>>(),
                              new Supplier<Set<String>>() {
                                  public Set<String> get() {
                                      return Sets.newSetFromMap(new ConcurrentHashMap<String, Boolean>());
                                  }
                              });
              mmap = concurrent ? Multimaps.synchronizedSetMultimap(multimap) : multimap;
              storeMap.put(index,mmap);
          }
          return mmap;
      }
docs: get or create the multimap object for the given {@code index}