def _MergeEventTag(self, storage_writer, attribute_container):
    """Merges an event tag with the last stored event tag.

    If there is an existing event the provided event tag is updated with
    the contents of the existing one. After which the event tag index is
    updated.

    Args:
      storage_writer (StorageWriter): storage writer.
      attribute_container (AttributeContainer): container.
    """
    if attribute_container.CONTAINER_TYPE != 'event_tag':
      return

    event_identifier = attribute_container.GetEventIdentifier()
    if not event_identifier:
      return

    # Check if the event has already been tagged on a previous occasion,
    # we need to append the event tag to the last stored one.
    stored_event_tag = self._event_tag_index.GetEventTagByIdentifier(
        storage_writer, event_identifier)
    if stored_event_tag:
      attribute_container.AddComment(stored_event_tag.comment)
      attribute_container.AddLabels(stored_event_tag.labels)

    self._event_tag_index.SetEventTag(attribute_container)
Merges an event tag with the last stored event tag.

    If there is an existing event the provided event tag is updated with
    the contents of the existing one. After which the event tag index is
    updated.

    Args:
      storage_writer (StorageWriter): storage writer.
      attribute_container (AttributeContainer): container.