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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 user_bindings(self):
 """ Access the user_bindings :returns: twilio.rest.chat.v2.service.user.user_binding.UserBindingList :rtype: twilio.rest.chat.v2.service.user.user_binding.UserBindingList """

        if self._user_bindings is None:
            self._user_bindings = UserBindingList(
                self._version,
                service_sid=self._solution['service_sid'],
                user_sid=self._solution['sid'],
            )
        return self._user_bindings