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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 GetEventTagByIdentifier(self, storage_file, event_identifier):
 """Retrieves the most recently updated event tag for an event. Args: storage_file (BaseStorageFile):
 storage file. event_identifier (AttributeContainerIdentifier):
 event attribute container identifier. Returns: EventTag: event tag or None if the event has no event tag. """

    if not self._index:
      self._Build(storage_file)

    lookup_key = event_identifier.CopyToString()
    event_tag_identifier = self._index.get(lookup_key, None)
    if not event_tag_identifier:
      return None

    return storage_file.GetEventTagByIdentifier(event_tag_identifier)