anchor.diff:    // refresh [fork] {create a new practice-session}
     //-----------------------------------------
   
  -  var refreshForkButton = function() {
  -    var button = $('#fork-button');
  -    cd.setTip(button, function() {
  -      var tip = 'setup a new session from ' + param.avatar + ' ' + param.nowTag;
  +  const refreshForkButton = () => {
  +    const button = $('#fork-button');
  +    cd.setTip(button, () => {
  +      const tip = 'setup a new session from ' + param.avatar + ' ' + param.nowTag;
         cd.showHoverTip(button, tip);
       });
     };
   
     // - - - - - - - - - - - - - - - - - - - - - - - -
   
  -  var doFork = function() {
  -
  -    var fork_params = {
  +  const doFork = () => {
  +    const forkParams = {
             id: param.id,
         avatar: param.avatar,
            tag: param.nowTag
       };
  -    $.getJSON('/forker/fork', fork_params, function(data) {
  +    $.getJSON('/forker/fork', forkParams, (data) => {
         if (data.forked) {
           cd.newDojoDialog(data);
         } else {
  
positive.code: 
    // - - - - - - - - - - - - - - - - - - - - - - - -
  
    var showFile = function(filenameId) {
      var filename =  $('#radio_' + filenameId);
      filename.click();
      filename.scrollIntoView({ direction: 'vertical' });
    };
  
    // - - - - - - - - - - - - - - - - - - - - - - - -
  
    var show = function(tag) {
      // preserve diff on/off state
      param.wasTag = tag - (diffCheckBox().is(':checked') ? 1 : 0);
      param.nowTag = tag;
  
negative.code: 
    // - - - - - - - - - - - - - - - - - - - - - - - -
  
    const showLatestNoDiff = () => {
      const lastTag = -1;
      param.wasTag = lastTag;
      param.nowTag = lastTag;
      refresh();
  