Cerrar menu desplegable

[pato12]

Hola,
Estuve haciendo un menu desplegable y anda de 10, pero queireo
que si se apretan en otra parte se cierra enves de volverlo a apretar.
Este es el código:

Código:

<style>
.menu_b_st {
	list-style-type: none;
	margin:0;
	text-decoration: none;
	float: left;
	font-size: 14px;
	border-top-width: 1px;
	border-bottom-width: 1px;
	border-left-width: 1px;
	border-top-style: solid;
	border-bottom-style: solid;
	border-left-style: solid;
	border-top-color: #999999;
	border-bottom-color: #999999;
	border-left-color: #999999;
}
.submenu_b_st {
	margin:-8px;
	list-style-type: none;
	display: none;
	position:absolute;
	background-color: #FFFFFF;
	color: #000000;
	border-top-width: 1px;
	border-right-width: 2px;
	border-bottom-width: 1px;
	border-left-width: 1px;
	border-top-style: solid;
	border-right-style: solid;
	border-bottom-style: solid;
	border-left-style: solid;
	border-top-color: #999999;
	border-right-color: #000000;
	border-bottom-color: #666666;
	border-left-color: #CCCCCC;
	height: auto;
	width: 100px;
	text-align: left;
	padding: 0px;
	cursor: default;
	float: left;
	font-size: 14px;
}
.submenu_b_st_ac{
	color:#000000;
	text-decoration: none;
	background-color: #C6D9FF;
	font-family: Arial, Helvetica, sans-serif;
	font-size: 14px;
}
.submenu_b_st_nod{
	color:#000000;
	text-decoration: none;
	background-color: #FFFFFF;
	font-family: Arial, Helvetica, sans-serif;
	font-size: 14px;
}
</style>
<script>
function $(id){ 
    return document.getElementById(id);     
}
function showlayer(layer,t){
if(!t)
t=false;
if($(layer).style.display=="none" || $(layer).style.display==""){
if(!t)
$(layer).style.display="block";
} else { 
$(layer).style.display="none";
}
}
function select_action(selec,p){
$('selected_t').innerHTML=selec;
$('select').value=p;
}
function selec_st_t(e,t){
if(t){
e.className="submenu_b_st_ac";
}else{
e.className="submenu_b_st_nod";
}
}
</script>
<ul class="menu_b_st">
  <li>
<samp onClick="showlayer('sub_menu');return false;" id="selected_t" style=" cursor:pointer;font-family:Arial, Helvetica, sans-serif;">Nuevo ▼
</samp>
<ul onClick="showlayer('sub_menu');" class="submenu_b_st" id="sub_menu">
  <li onmousemove="selec_st_t(this,true);" onmouseout="selec_st_t(this,false);" onClick="select_action('Nuevo &#9660','nuv');return false;">Nuevo</li>
  <li onmousemove="selec_st_t(this,true);" onmouseout="selec_st_t(this,false);" onClick="select_action('Borrar &#9660','bor');return false;">Borrar</li>
  <li onmousemove="selec_st_t(this,true);" onmouseout="selec_st_t(this,false);" onClick="select_action('Editar &#9660','edt');return false;">Editar</li>
</ul>
</li>
</ul><input name="select" id="select" type="hidden" value="nuv" />

Gracias
Salu2