anchor.diff:  
   <div class="tab-filenames">
     <div class="tab output">output</div>
  -  <div class="tab filename">cyber-dojo.sh</div>
  +  <div class="tab filename"></div>
     <div class="tab REPL">+REPL</div>
   </div>
   
  
positive.code:   tabs.reserves = filename => {
      const tabNames = [];
      tabNames.push(tabs.output().text());
      tabNames.push('status');
      tabNames.push(tabs.repl().text());
      return tabNames.includes(filename);
    };
  
negative.code:     // Alt-O tabs hot-key navigation
      // See app/assets/javascripts/cyber-dojo_codemirror.js
      // See app/views/shared/_hotkeys.html.erb
      const text = tabs.selected().text();
      if (text === 'output') {
        tabs.filename().click();
      } else {
        tabs.output().click();
      }
    };
  
  