function zebra_table(id) {
    if (!document.getElementById) return;
    var even = false;

    var table = document.getElementById(id);
    if (! table) { return; }

    var tbodies = table.getElementsByTagName("tbody");

    for (var h = 0; h < tbodies.length; h++) {
        var trs = tbodies[h].getElementsByTagName("tr");

        for (var i = 0; i < trs.length; i++) {

            if (even) {
                add_class(trs[i], 'even');
            } else {
                add_class(trs[i], 'odd');
            }
            even =  ! even;
        }
    }
}

// from y!
// add a class to an element
function add_class (element, class_name) {
    // do not add duplicates
    if (element && element.className.indexOf(class_name) == -1)
        element.className += " " + class_name;
}


// remove a class from an element
function remove_class (element, class_name) {
    if (!element) return;

    element.className = element.className.replace(class_name, "");
    // this leaves ll those " " behind, no harm
}