anchor.diff:   * IEEE Computer Society, pp. 73-78, 2001.
    **/
   class Pivot extends LayoutAlgorithm with LayoutUtils {
  -  
  -  static PivotSelectionStrategy bySize = 
  -      (Iterable<DataModel> models) => models.reduce((value, element) {
  -        if (element.size > value.size) {
  -          value = element;
  -        }
  -        return value;
  -      });
  -  
  -  static PivotSelectionStrategy byMiddle = 
  -      (Iterable<DataModel> models) => models.elementAt(models.length ~/ 2);
  -  
  +    
     PivotSelectionStrategy _selectionStrategy;
     
     Pivot([PivotSelectionStrategy selectionStrategy]) {
  
positive.code:     throw new UnsupportedError("Not needed for pivot algorithm");    
    }
    
  }
negative.code: part of treemap_ui.layout;
  
  typedef DataModel PivotSelectionStrategy(Iterable<DataModel> models);
  
  PivotSelectionStrategy bySize = 
    (Iterable<DataModel> models) => models.reduce((value, element) {
      if (element.size > value.size) {
        value = element;
      }
      return value;
    });
  
  PivotSelectionStrategy byMiddle = 
    (Iterable<DataModel> models) => models.elementAt(models.length ~/ 2);
  
  /**
   * Implementation of the pivot layout algorithm. For further details see
  