document.getElementsByClassName = function(class_name)
{
    var result = [];
    var class_match = new RegExp('\\b' + class_name + '\\b');
    var elements = this.getElementsByTagName('*');
    for (var i = 0; i != elements.length; ++i)
    {
        var name = elements[i].className;
        if (class_match.test(name))
            result.push(elements[i]);
    }
    return result;
}

function on_tab_select(id)
{
    var current = document.all[id].style.display;
    if (current == 'none') current = '';
    var tabs = document.getElementsByClassName('tab_content')
    for (var i = 0; i != tabs.length; ++i)
        tabs[i].style.display = '';
    document.all[id].style.display = (current == '') ? 'block' : ''
}