クリックしたら入れ子のUL要素の表示・非表示を切り替えるJavaScript

var open_child_ul = function(e){
	var ul = e.nextSibling;
	while(ul.tagName != "UL"){
		ul = ul.nextSibling;
	}
	ul.style.display="block";
	e.onclick=function(){close_child_ul(this)}
}
var close_child_ul = function(e){
	var ul = e.nextSibling;
	while(ul.tagName != "UL"){
		ul = ul.nextSibling;
	}
	ul.style.display="none";
	e.onclick=function(){open_child_ul(this)}
}