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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 _get_server(self, server_id):
 """ Internal method to get the server object, given a server_id. Parameters: server_id (:term:`string`):
 The server ID of the WBEM server, returned by :meth:`~pywbem.WBEMSubscriptionManager.add_server`. Returns: server (:class:`~pywbem.WBEMServer`):
 The WBEM server. Raises: ValueError: server_id not known to subscription manager. """


        if server_id not in self._servers:
            raise ValueError(
                _format("WBEM server {0!A} not known by subscription manager",
                        server_id))

        return self._servers[server_id]