code: def add_payload(self, payload):
          
          if self._payload is None:
              self.decode_payload()
          if len(self._payload) >= 1:
              raise ValueError("Cannot add more payload to Iq stanza")
          return Stanza.add_payload(self, payload)
docs: Add new the stanza payload. Fails if there is already some
          payload element attached (<iq/> stanza can contain only one payload
          element)
  
          Marks the stanza dirty.
  
          :Parameters:
              - `payload`: XML element or stanza payload object to add
          :Types:
              - `payload`: :etree:`ElementTree.Element` or
                `interfaces.StanzaPayload`