function toggle(id, level) {
	var children = document.getElementById("children_" + id);
	var parent   = document.getElementById("parent_" + id);
	var visibility = children.style.display;
	if (visibility == 'none') {
		children.style.display = 'block';
		parent.style.backgroundImage = "url(/images/button"+level+"_open.jpg)";
	} else {
		children.style.display = 'none';
		parent.style.backgroundImage = "url(/images/button"+level+"_close.jpg)";
	}
}