code: def registration_form_received(self, stanza):
          
          self.lock.acquire()
          try:
              self.__register = Register(stanza.get_query())
              self.registration_callback(stanza, self.__register.get_form())
          finally:
              self.lock.release()
docs: Handle registration form received.
  
          [client only]
  
          Call self.registration_callback with the registration form received
          as the argument. Use the value returned by the callback will be a
          filled-in form.
  
          :Parameters:
              - `stanza`: the stanza received.
          :Types:
              - `stanza`: `pyxmpp.iq.Iq`