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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 function_versions(self):
 """ Access the function_versions :returns: twilio.rest.serverless.v1.service.function.function_version.FunctionVersionList :rtype: twilio.rest.serverless.v1.service.function.function_version.FunctionVersionList """

        if self._function_versions is None:
            self._function_versions = FunctionVersionList(
                self._version,
                service_sid=self._solution['service_sid'],
                function_sid=self._solution['sid'],
            )
        return self._function_versions