y event handlers to toggle the state of the checkbox
 *
 * @param ( Checkbox object ) checkbox  Checkbox to toggle state
 *
 * @return none
 */

toggleCheckbox = function( checkbox ) {

  if (checkbox.node.getAttribute("aria-checked") == "true") {

    // If the checkbox is currently checked set the state to unchecked	

    checkbox.node.setAttribute("aria-checked", "false");
    checkbox.image_node.src= "images/unchecked.gif";

  } else {

    // If the checkbox is currently unchecked set the state to chec