*     for a given row key
   * @throws IllegalArgumentException if {@code backingMap} is not empty
   * @since 10.0
   */
  public static <R, C, V> Table<R, C, V> newCustomTable(
      Map<R, Map<C, V>> backingMap, Supplier<? extends Map<C, V>> factory) {
    checkArgument(backingMap.isEmpty());
    checkNotNull(factory);