function tree_toggle(event) {
    event = event || window.event
    var clickedElem = event.target || event.srcElement
    if ((!hasClass(clickedElem, 'Expand'))) {
        return
    }

    var node = clickedElem.parentNode
    var newClass = hasClass(node, 'ExpandOpen') ? 'ExpandClosed' : 'ExpandOpen'
    var re =  /(^|\s)(ExpandOpen|ExpandClosed)(\s|$)/
    node.className = node.className.replace(re, '$1'+newClass+'$3')
}
function hasClass(elem, className) {
    return new RegExp("(^|\\s)"+className+"(\\s|$)").test(elem.className)
}