code: function() {
      this._destroyDraggableColumns();
      if(util.DD) {
          var oColumn, elTh, elDragTarget;
          for(var i=0, len=this._oColumnSet.tree[0].length; i<len; i++) {
              oColumn = this._oColumnSet.tree[0][i];
              elTh = oColumn.getThEl();
              Dom.addClass(elTh, DT.CLASS_DRAGGABLE);
              elDragTarget = DT._initColumnDragTargetEl();
              oColumn._dd = new YAHOO.widget.ColumnDD(this, oColumn, elTh, elDragTarget);
          }
      }
      else {
          YAHOO.log("Could not find DragDrop for draggable Columns", "warn", this.toString());
      }
  }
docs: Initializes top-level Column TH elements into DD instances.
  
  @method _initDraggableColumns
  @private