docs: def _login(self, credentials):
          """
          Actually login to our portal with the given credentials.
          """
          d = self.portal.login(credentials, None, IBoxReceiver)
          def cbLoggedIn((interface, avatar, logout)):
              self.logout = logout
              self.boxReceiver = avatar
              self.boxReceiver.startReceivingBoxes(self.boxSender)
              return {}
          d.addCallback(cbLoggedIn)
          return d