docs: @Beta
    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);
      // TODO(jlevy): Wrap factory to validate that the supplied maps are empty?
      return new StandardTable<R, C, V>(backingMap, factory);
    }