Accordion - Collapsable Panels - NESTED - SINGLE OPEN - Bootstrap 3

HTML

<link rel="stylesheet" href="http://netdna.bootstrapcdn.com/bootstrap/3.0.0/css/bootstrap.min.css">
<script src="http://netdna.bootstrapcdn.com/bootstrap/3.0.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="http://netdna.bootstrapcdn.com/font-awesome/3.2.1/css/font-awesome.min.css">
	<!-------------------------------   PANEL AGENZIA DELLE ENTRATE    ----------------------------------->
	<div class="panel-group" id="accordion0">
	<div class="panel panel-default">
		<!-- a1-->
		<div class="panel-heading">
			 <h4 class="panel-title">
				 <a data-toggle="collapse" data-parent="#accordion0" href="#collapse1">
				 Come importare la lista degli immobili per codice fiscale dal sito dell'Agenzia delle Entrate?
				 </a>
			 </h4>
		</div>
		<div id="collapse1" class="panel-collapse collapse ">
			<div class="panel-body">
			
				<div class="panel-group" id="accordion1">
				
					<div class="panel panel-default">
						<div class="panel-heading">
							 <h4 class="panel-title">
								<a data-toggle="collapse" data-parent="#accordion1" href="#collapse11">
									Visualizza la tabella degli immobili sul sito dell'Agenzia
								</a>
							  </h4>
						</div>
						<div id="collapse11" class="panel-collapse collapse">
							<div class="panel-body">Visualizza la tabella degli immobili sul sito dell'Agenzia Content</div>
						</div>
					</div>
					<div class="panel panel-default">
						<div class="panel-heading">
							<h4 class="panel-title">
								<a data-toggle="collapse" data-parent="#accordion1" href="#collapse12">            
								   Importazione con Estensione Chrome/Firefox
								</a>
							</h4>
						</div>
						<div id="collapse12" class="panel-collapse collapse">
							<div class="panel-body">Importazione con Estensione Chrome/Firefox content</div>
						</div>
					</div>
					<div class="panel panel-default">
						<div class="panel-heading">
							<h4 class="panel-title">
								<a data-toggle="collapse"...