anchor.diff:      // Alt-O tabs hot-key navigation
       // See app/assets/javascripts/cyber-dojo_codemirror.js
       // See app/views/shared/_hotkeys.html.erb
  -    switch ($selectedTab().text()) {
  +    switch (tabs.selected().text()) {
       default                : tabs.stdout().click(); break;
       case 'stdout'          : tabs.stderr().click(); break;
       case 'stderr'          : tabs.status().click(); break;
  
positive.code:     // Called from filenames. filename can be
      //  o) a true filename (but never '+REPL')
      //  o) 'stdout', 'stderr', 'status'
      if ($selectedTab().text() === tabs.repl().text()) {
        cd.kata.repl.close();
      }
      const id = cd.isOutputFile(filename) ? `radio_${filename}` : 'current_filename';
  
negative.code:   tabs.stdout   = () => $('#radio_stdout' , $tabs);
    tabs.stderr   = () => $('#radio_stderr' , $tabs);
    tabs.status   = () => $('#radio_status' , $tabs);
    tabs.selected = () => $('.selected', $tabs);
  
    //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  
    const $tabs = $('.tab-filenames');
    const alreadySelected = $tab => $tab.hasClass('selected');
    const select = $tab => {
      $('.tab-filename').removeClass('selected');
  