function on(cosa) {
        var lt = document.getElementById("lt_" + cosa);
        var rt = document.getElementById("rt_" + cosa);
        lt.style.visibility = 'visible';
        rt.style.visibility = 'visible';
    }
    function off(cosa) {
        var lt = document.getElementById("lt_" + cosa);
        var rt = document.getElementById("rt_" + cosa);
        lt.style.visibility = 'hidden';
        rt.style.visibility = 'hidden';
    }