<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _write_version(self, data, model):
 """ Writes a copy of the objects current state to write-once mirror bucket. Args: data (dict):
 Model instance's all data for versioning. model (instance):
 Model instance. Returns: Key of version record. key (str):
 Version_bucket key. """

        vdata = {'data': data,
                 'key': model.key,
                 'model': model.Meta.bucket_name,
                 'timestamp': time.time()}
        obj = version_bucket.new(data=vdata)
        obj.add_index('key_bin', model.key)
        obj.add_index('model_bin', vdata['model'])
        obj.add_index('timestamp_int', int(vdata['timestamp']))
        obj.store()
        return obj.key