function montre(id){
if(id=="a2"){
document.getElementById('back').style.visibility="visible";
document.getElementById('fw').style.visibility="hidden";
}
else{
document.getElementById('back').style.visibility="hidden";
document.getElementById('fw').style.visibility="visible";
}
	if (document.getElementById(id).style.display=="block"){
	document.getElementById(id).style.display="none";
	}
	else {
		for (var i=1; i<3; i++){
		document.getElementById("a"+i).style.display="none";
		}
	document.getElementById(id).style.display="block";
	}
}