function show_ag_description(id, a) {
    var descriptions = document.getElementById('descriptions').getElementsByTagName('div');
    for (i=0; i < descriptions.length; i++) {
        descriptions[i].style.display = 'none';
    }
    document.getElementById(id).style.display = 'block';
    var article_groups = document.getElementById('article_groups').getElementsByTagName('li');
    for (i=0; i < article_groups.length; i++) {
        link = article_groups[i].getElementsByTagName('a')[0];
        link.className = '';
    }
    a.className = 'active';
}