function news() {
    if (document.getElementById('nav01').style.visibility == 'hidden') {
        document.getElementById('nav01').style.visibility = 'visible';
        document.getElementById('nav02').style.visibility = 'hidden';
        document.getElementById('nav03').style.visibility = 'hidden';
    }
}
function forum() {
    if (document.getElementById('nav02').style.visibility == 'hidden') {
        document.getElementById('nav01').style.visibility = 'hidden';
        document.getElementById('nav02').style.visibility = 'visible';
        document.getElementById('nav03').style.visibility = 'hidden';
    }
}
function blog() {
    if (document.getElementById('nav03').style.visibility == 'hidden') {
        document.getElementById('nav01').style.visibility = 'hidden';
        document.getElementById('nav02').style.visibility = 'hidden';
        document.getElementById('nav03').style.visibility = 'visible';
    }
}