additional layer of felt的用法和样例：
- The inside is lined with another layer of felt to keep out the cold.
- Users are discovering that Web caches can solve both problems and provide an additional layer of defense against hackers.
- Because the front-end server has no user information stored on it, it provides an additional layer of security for the organization.
- Investments through third parties, such as hedge funds, help too, providing an additional layer of protection against the misdeeds of rogues.