anchor.diff:  
     cd.setFilenameTab = (filename) => {
       // filename = from filename-knave, stdout, stderr, status (never +REPL)
  -    if (selectedTabFilename() === $replTab().text()) {
  +    if ($selectedTab().text() === $replTab().text()) {
         cd.closeRepl();
       }
       const id = cd.isOutputFile(filename) ? `radio_${filename}` : 'current_filename';
  
positive.code:   const setupReplTabClickHandler = ($tab) => {
      $tab.click(() => {
        if (alreadySelected($tab)) { return; }
        cd.unloadFile(selectedTabFilename());
        cd.openRepl();
        select($tab);
      });
  
negative.code:   // Alt-O ==> toggleOutputFile()
  
    cd.toggleOutputFile = () => {
      switch ($selectedTab().text()) {
      default               : cd.loadFile('stdout'); break;
      case 'stdout'         : cd.loadFile('stderr'); break;
      case 'stderr'         : cd.loadFile('status'); break;
  