function onCheckBoxClick(chk) {
          var combo = $find("<%= rcbDocType.ClientID %>");
          //holds the text of all checked items
          var text = "";
          //holds the values of all checked items
          var values = "";
          //get the collection of all items
          var items = combo.get_items();
          //enumerate all items
          for (var i = 0; i < items.get_count(); i++) {
              var item = items.getItem(i);
              //get the checkbox element of the current item
              var chk1 = $get(combo.get_id() + "_i" + i + "_cbItem");
              if (chk1.checked) {
                  if (text == "") {
                      text = item.get_text();
                  }
                  else {
                      text = "[Multi]";
                  }
                  values += item.get_value() + ",";
              }
          }
          //remove the last comma from the string
          values = removeLastComma(values);

          if (text.length > 0) {
              //set the text of the combobox
              combo.set_text(text);
          }
          else {
              //all checkboxes are unchecked
              //so reset the controls       
              combo.set_text("");
          }
      }

      //this method removes the ending comma from a string
      function removeLastComma(str) {
          return str.replace(/,$/, "");
      }

      function StopPropagation(e) {
          //cancel bubbling
          e.cancelBubble = true;
          if (e.stopPropagation) {
              e.stopPropagation();
          }
      }