code: function retro(content) {
      // Lex incoming markdown file
      var lexed = lex(content);
  
      // Break it into sections
      var newSections = sections.split(lexed)
      .map(function(section) {
          // Leave non quiz sections untouched
          if(!isQuiz(section)) {
              return section;
          }
          // Convert old section to new format
          return retroSection(section);
      });
  
      // Bring sections back together into a list of lexed nodes
      var newLexed = sections.join(newSections);
  
      // Render back to markdown
      return render(newLexed);
  }
docs: retro takes an input markdown file and outputs a new markdown file converting in the process old quizs to new ones