I often have the need to let a program just call an object to encode/decode a payload, without fussing about keys, algorithms etc.
The 'coder' object can be prepared and then given to a consumer that only has to call decode/encode.
This mini interface provides that functionality and can be used for JWS, JWE and Nested Webtoken.
It need an implementation, e.g. for web-token/jwt.