docs: def send_signal(self, backend, signal):
          """
          Sends the `signal` signal to `backend`. Raises ValueError if `backend` 
          is not registered with the client. Returns the result.
          """
          backend = self._expand_host(backend)
          if backend in self.backends:
              try:
                  return self._work(backend, self._package(signal), log=False)
              except socket.error:
                  raise BackendNotAvailableError
          else:
              raise ValueError('No such backend!')