code: def dump(self):
          
          result = list(self.output_lines())
          if self.locked:
              result.append('# locked')
              if self.choices:
                  for choice in self.choices:
                      result.append('# choice: ' + choice)
          if self.explanation:
              result.append('# explanation: ' + self.explanation)
          return '\n'.join(result)
docs: Serialize a test case to a string.