'Locks a specific subject.'

def LockSubject(self, subject, lease_time, token):

    request = rdf_data_store.DataStoreRequest(subject=[subject])
    specific = rdf_data_store.TimestampSpec.Type.SPECIFIC_TIME
    request.timestamp = rdf_data_store.TimestampSpec(start=lease_time, type=specific)
    if token:
        request.token = token
    typ = rdf_data_server.DataStoreCommand.Command.LOCK_SUBJECT
    response = self._MakeSyncRequest(request, typ)
    if (not response.results):
        return None
    result = response.results[0]
    if (not result.values):
        return None
    return result.values[0].value.string
