var last;

function show_menu(id) {
	if (id == last) {
		document.getElementById('menu_'+id).style.visibility = 'hidden';
		last = '';
	}
	else {
		if (last) {
			document.getElementById('menu_'+last).style.visibility = 'hidden';
		}
		document.getElementById('menu_'+id).style.visibility = 'visible';
		last = id;	
	}
}