old_contents: class Formatter():
      pass
  
  
  class QuestionFormatter(Formatter):
      pass
  
  
  class QuizFormatter(Formatter):
      def to_string(self, quiz, header="", footer=""):
          for i, q in enumerate(quiz.iter_questions()):
              if i == 0:
                  s = q.to_string()
              else:
                  s += "\n\n" + q.to_string()
          if header != "":
              s = header + "\n" + s
          if footer != "":
              s = s + "\n" + footer
          return s
  
new_contents: class Formatter():
      pass
  
  
  class QuestionFormatter(Formatter):
      pass
  
  
  class QuizFormatter(Formatter):
      def to_string(self, quiz, header="", footer=""):
          s = ""
          for i, q in enumerate(quiz.iter_questions()):
              if i == 0:
                  s = q.to_string()
              else:
                  s += "\n\n" + q.to_string()
          if header != "":
              s = header + "\n" + s
          if footer != "":
              s = s + "\n" + footer
          return s
  
subject: Fix uncatched error