code: def extensions(self):
          
          if self._extensions is None:
              self._extensions = AvailableAddOnExtensionList(
                  self._version,
                  available_add_on_sid=self._solution['sid'],
              )
          return self._extensions
docs: Access the extensions
  
          :returns: twilio.rest.preview.marketplace.available_add_on.available_add_on_extension.AvailableAddOnExtensionList
          :rtype: twilio.rest.preview.marketplace.available_add_on.available_add_on_extension.AvailableAddOnExtensionList