var last_id = "intro";
function show(id_name) {
	// hide last div
	if (last_id) {
		document.getElementById('link_'+last_id).className = "off";
		document.getElementById('year_'+last_id).className = "hide";
	}
	//if value of the box is not nothing and an object with that name exists, then change the class
	if (id_name && document.getElementById('year_'+id_name)) {
		document.getElementById('link_'+id_name).className = "current";
		document.getElementById('year_'+id_name).className = "show";
		last_id = id_name;
	}
}