function hide_object( identifier ){
    var obj;
    if( document.getElementById( identifier ) ){
        obj = document.getElementById( identifier );
        obj.setAttribute('class', 'hidden');    
    }
}

function show_object( identifier ){
    var obj;
    if( document.getElementById( identifier ) ){
        obj = document.getElementById( identifier );
        obj.setAttribute('class', 'visible');    
    }
}