anchor.diff:    final String name;
     final String subject;
     final String body;
  +
  +  @override
  +  int compareTo(LeaveBehindItem other) => index.compareTo(other.index);
   }
   
   class LeaveBehindDemo extends StatefulWidget {
  
positive.code:   }
  
    void handleUndo(LeaveBehindItem item) {
      int insertionIndex = lowerBound(leaveBehindItems, item,
        compare: (LeaveBehindItem a, LeaveBehindItem b) => a.index.compareTo(b.index)
      );
      setState(() {
        leaveBehindItems.insert(insertionIndex, item);
      });
  
negative.code:   rightSwipe
  }
  
  class LeaveBehindItem implements Comparable<LeaveBehindItem> {
    LeaveBehindItem({ this.index, this.name, this.subject, this.body });
  
    LeaveBehindItem.from(LeaveBehindItem item)
  