code: def AddEventTag(self, event_tag):
      
      self._RaiseIfNotWritable()
  
      event_identifier = event_tag.GetEventIdentifier()
      if not isinstance(event_identifier, identifiers.FakeIdentifier):
        raise IOError('Unsupported event identifier type: {0:s}'.format(
            type(event_identifier)))
  
      event_tag = self._PrepareAttributeContainer(event_tag)
  
      self._event_tags.append(event_tag)
      self.number_of_event_tags += 1
docs: Adds an event tag.
  
      Args:
        event_tag (EventTag): event tag.
  
      Raises:
        IOError: when the storage writer is closed.
        OSError: when the storage writer is closed.