public static @NotNull <K, V> MultiMap<K, V> createConcurrentSet() {
    return new MultiMap<K, V>(new ConcurrentHashMap<>()) {
      @Override
      protected @NotNull Collection<V> createCollection() {
        return ContainerUtil.newConcurrentSet();
      }

      @Override
      protected @NotNull Collection<V> createEmptyCollection() {
        return Collections.emptySet();
      }
    };
  }
public static @NotNull <K, V> MultiMap<K, V> createConcurrentSet()