 /// type identifiers for dragging operation
 private enum PboardType {
     static let rows = "rows"
-    static let objects = "objects"
 }
 
 

    func tableView(_ tableView: NSTableView, writeRowsWith rowIndexes: IndexSet, to pboard: NSPasteboard) -> Bool {
        
        // register dragged type
        tableView.register(forDraggedTypes: [PboardType.rows, PboardType.objects])
        pboard.declareTypes([PboardType.rows, PboardType.objects], owner: self)
        
        // select rows to drag
        tableView.selectRowIndexes(rowIndexes, byExtendingSelection: false)

        // store row index info to pasteboard
        let rows = NSKeyedArchiver.archivedData(withRootObject: rowIndexes)
        pboard.setData(rows, forType: PboardType.rows)
        
        return true
    }
