Redefining the same method signature is an option, but I'm a bit reluctant to it. However, it would be good to capture in an interface what is linked to logout...

So far, we have :
interface : ```Principal``` / methods : ```getId```, ```getAttributes```
interface : ```Service``` inherits from ```Principal``` (that's not clear to me why ?) / methods : ```setPrincipal```, ```matches```
interface : ```WebApplicationService``` inherits from ```Service``` / methods : ```getResponse``` / ```getArtifactId```

So I propose to add the ```getOriginalUrl``` method to the ```WebApplicationService``` interface and finally create a ```SingleLogoutService``` inheriting from ```WebApplicationService```...