code: def add_backend(self, backend):
          "Add a RapidSMS backend to this tenant"
          if backend in self.get_backends():
              return
          backend_link, created = BackendLink.all_tenants.get_or_create(backend=backend)
          self.backendlink_set.add(backend_linkf add_backend(self, backend):
          "Add a RapidSMS backend to this tenant"
          if backend in self.get_backends():
              return
          backend_link, created = BackendLink.all_tenants.get_or_create(backend=backend)
          self.backendlink_set.add(backend_link)
docs: Add a RapidSMS backend to this tenant