var navObj = document.all.menu_nav;
	var aaObj = document.all.aa;
	
	function change_layer(index){
		
		for(var i = 0 ; i < aaObj.length ; i ++){
			
			aaObj[i].style.display="none";
		}
		aaObj[index].style.display="";
		
	}
	change_layer(0);