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!')
Sends the `signal` signal to `backend`. Raises ValueError if `backend` 
        is not registered with the client. Returns the result.