anchor.diff:    @override
     SelectableGetter<T> getSelectable;
   
  -  /// Whether or not the concept of "all entities" is applicable to this model
  -  /// or not.
  -  ///
  -  /// Useful for cases such as an unknown number of entities in the model
  -  /// due to expansion.
  -  bool supportsSelectAll;
  -
     /// If the table supports selection over multiple pages.
     bool supportsMultiplePageSelection;
   
  
positive.code: import 'selection_model.dart';
  
  typedef bool IsIndeterminate(var entity);
  
  /// Count getter to return when there is no valid count
  final CountGetter _noCountGetter = () => -1;
  
  /// Selection model interface that is meant to be used with the table view
  /// component.
  
negative.code:   // allOnPageSelected and allAcrossPagesSelected, but it turns out to be
    // ambiguous and does more bad than good.
    bool get allAcrossPagesSelected =>
        _allSelected ||
        (_selectedCount > 0 && totalEntitiesCount() == _selectedCount);
  
    @override
    void selectAllAcrossPages() {
      if (_allSelected) return;
      assert(supportsMultiplePageSelection);
      bool wasEmpty = isEmpty;
      _allSelected = true;
      if (wasEmpty) {
  