<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def SetEventTag(self, event_tag):
 """Sets an event tag in the index. Args: event_tag (EventTag):
 event tag. """

    event_identifier = event_tag.GetEventIdentifier()

    lookup_key = event_identifier.CopyToString()
    self._index[lookup_key] = event_tag.GetIdentifier()