code: def nest(self, verb):
          
          if not isinstance(verb, TwiML) and not isinstance(verb, str):
              raise TwiMLException('Only nesting of TwiML and strings are allowed')
  
          self.verbs.append(verb)
          return verb
docs: Add a TwiML doc. Unlike `append()`, this returns the created verb.
  
          :param verb: TwiML Document
  
          :returns: the TwiML verb