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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 tokens(self):
 """ Access the tokens :returns: twilio.rest.api.v2010.account.token.TokenList :rtype: twilio.rest.api.v2010.account.token.TokenList """

        if self._tokens is None:
            self._tokens = TokenList(self._version, account_sid=self._solution['sid'], )
        return self._tokens