function checkboxlist(checkbox){

    // If a checkbox option from a session track is selected, make sure
    // the rest are unchecked as only one item can be selected per session.
    if (checkbox.checked) {
    
        var checkboxes = checkbox.parentNode.getElementsByTagName('input');

        // Uncheck all the checkboxes except for the selected one.        
        for (var i=0; i < checkboxes.length; i++) {
            if (checkbox != checkboxes[i]) {
                checkboxes[i].checked = false;
            }      
        }
    }
    
}