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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_service_reference(self, clazz, ldap_filter=None):
 # type: (Optional[str], Optional[str]) -> Optional[ServiceReference] """ Returns a ServiceReference object for a service that implements and was registered under the specified class :param clazz: The class name with which the service was registered. :param ldap_filter: A filter on service properties :return: A service reference, None if not found """

        result = self.__framework.find_service_references(
            clazz, ldap_filter, True
        )
        try:
            return result[0]
        except TypeError:
            return None