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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 team_abbreviation(self):
 """ Returns a ``string`` of the team's abbreviation, such as 'DET' for the Detroit Red Wings. """

        # For career stats, skip the team abbreviation.
        if self._season[self._index].lower() == 'career':
            return None
        return self._team_abbreviation[self._index]