function tree_control (varObj) {
	ul    = varObj.parentNode.parentNode.childNodes.item(1);
	img   = varObj;
	disp_str = ul.style.display;
	switch (disp_str) {
		case 'none':
			style_str = 'block';
			img.setAttribute('src','../img/arrow-open.gif');
			break;
		case 'block':
			style_str = 'none';
			img.setAttribute('src','../img/arrow-close.gif');
			break;
	}
	ul.style.display = style_str;
}