code: def extensions(self):
          
          if self._extensions is None:
              self._extensions = InstalledAddOnExtensionList(
                  self._version,
                  installed_add_on_sid=self._solution['sid'],
              )
          return self._extensions
docs: Access the extensions
  
          :returns: twilio.rest.preview.marketplace.installed_add_on.installed_add_on_extension.InstalledAddOnExtensionList
          :rtype: twilio.rest.preview.marketplace.installed_add_on.installed_add_on_extension.InstalledAddOnExtensionList