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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def signal_stop(self, mode):
 """Signal postmaster process to stop :returns None if signaled, True if process is already gone, False if error """

        if self.is_single_user:
            logger.warning("Cannot stop server; single-user server is running (PID: {0})".format(self.pid))
            return False
        try:
            self.send_signal(STOP_SIGNALS[mode])
        except psutil.NoSuchProcess:
            return True
        except psutil.AccessDenied as e:
            logger.warning("Could not send stop signal to PostgreSQL (error: {0})".format(e))
            return False

        return None