* multimap must support null keys and values.
   */
  private static void checkUnmodifiableMultimap(
      Multimap<@Nullable String, @Nullable Integer> multimap, boolean permitsDuplicates) {
    checkUnmodifiableMultimap(multimap, permitsDuplicates, null, null);
  }

  /**
   * The supplied multimap will be mutated and an unmodifiable instance used in its stead. If the