function show_menu(i,j){
		hide_menu(j);
		var sub_menu_id = "sub_menu_"+i;
		document.getElementById(sub_menu_id).style.visibility = "visible";
		var sub_link_id = "sub_link_0";
		document.getElementById(sub_link_id).style.visibility = "hidden";
	}
	
	function show_sub_link(i,j){
		hide_menu(j);
		document.getElementById("sub_menu_7").style.visibility = "visible";
		var sub_link_id = "sub_link_"+i;
		document.getElementById(sub_link_id).style.visibility = "visible";
	}
	
	function show_sub_link_student(j){
		hide_menu(j);
		document.getElementById("sub_link_0").style.visibility = "visible";
	}
	
	function hide_menu(j){
		for(var i = 0; i < 10; i ++){
			var sub_menu_id = "sub_menu_"+i;
			document.getElementById(sub_menu_id).style.visibility = "hidden";
		}
		for(var i = 0; i < j; i ++){
			var sub_menu_id = "sub_link_"+i;
			document.getElementById(sub_menu_id).style.visibility = "hidden";
			var sub_menu_id = "kon_link_"+i;
			document.getElementById(sub_menu_id).style.visibility = "hidden";
		}
	}
	
	function change_font_size(){
		var font_size = document.getElementsByName("font_size")[0].value;
		document.getElementById("content").style.fontSize = font_size+"px";
	}
	
	function change_font_color(){
		var font_color = document.getElementsByName("font_color")[0].value;
		document.getElementById("content").style.color = font_color;
	}
	
	function show_zone(id){
		hide_zone();
		var zone_id2show = "zone_"+id;
		document.getElementById(zone_id2show).style.visibility = "visible";
	}
	
	function hide_zone(){
		for(var i = 1; i < 6; i ++){
			var zone_id = "zone_"+i;
			document.getElementById(zone_id).style.visibility = "hidden";
		}
	}