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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 users(self):
 """ Access the users :returns: twilio.rest.chat.v2.service.user.UserList :rtype: twilio.rest.chat.v2.service.user.UserList """

        if self._users is None:
            self._users = UserList(self._version, service_sid=self._solution['sid'], )
        return self._users