function flip(id) {
    if(document.getElementById(id).style.display == "block") {
        document.getElementById(id).style.display="none";
        document.getElementById(id + "_button").src="/images/icon/bullet_toggle_plus.png";
    } else {
        document.getElementById(id).style.display="block";
        document.getElementById(id + "_button").src="/images/icon/bullet_toggle_minus.png";
    }
}