code: def cannot_convert(self, node, reason=None):
          
          lineno = node.get_lineno()
          for_output = node.clone()
          for_output.prefix = u""
          msg = "Line %d: could not convert: %s"
          self.log_message(msg % (lineno, for_output))
          if reason:
              self.log_message(reason)
docs: Warn the user that a given chunk of code is not valid Python 3,
          but that it cannot be converted automatically.
  
          First argument is the top-level node for the code in question.
          Optional second argument is why it can't be converted.