function toggleFolder( id, makeVisible ) {

  // perform visibility toggle

  var btn = document.getElementById( "folderToggleButton" + id );
  var content = document.getElementById( "folderContent" + id );

  // see if we're forcing visibility

  if (makeVisible && isVisible(content)) { return; }

  // perform toggle

  toggleVisibility( content );

  if (isHidden(content)) {
    folderState[id] = false;
    btn.innerText = "+";
  } else {
    folderState[id] = true;
    btn.innerText = "-";
  }

  // serialize folder state

  var folderStateSerialized = "";

  for( s in folderState ) {
    if (folderState[s] == true) {
      folderStateSerialized += ",";
      folderStateSerialized += s
    }
  }

  setCookie( "folderStateSerialized", folderStateSerialized.substr(1), null, '/' );

}