}

  /**
   * Returns the empty multimap.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */
  // Casting is safe because the multimap will never hold any elements.
  @SuppressWarnings("unchecked")
  public static <K, V> ImmutableSetMultimap<K, V> of() {
    return (ImmutableSetMultimap<K, V>) EmptyImmutableSetMultimap.INSTANCE;