* {@code map} and the instances generated by {@code factory} are. Concurrent read operations will
   * work correctly. To allow concurrent update operations, wrap the multimap with a call to {@link
   * #synchronizedListMultimap}.
   *
   * <p>Call this method only when the simpler methods {@link ArrayListMultimap#create()} and {@link
   * LinkedListMultimap#create()} won't suffice.
   *