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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 validation_requests(self):
 """ Access the validation_requests :returns: twilio.rest.api.v2010.account.validation_request.ValidationRequestList :rtype: twilio.rest.api.v2010.account.validation_request.ValidationRequestList """

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