var last_id = null;
function click_menu(id){
 
 	el=document.getElementById(id);
	
	if (last_id != null && last_id != '' && last_id != id){
		if (document.getElementById(last_id) != 'undefined'){
			document.getElementById(last_id).className = 'invisible';
		}
	}

	if (el.className=='invisible') {
		el.className='visible';
		last_id = id;
	}
	else {
		el.className='invisible';
		last_id = '';
	}
}