Just one function for sign up and log in?

[timbl]

"Both Login and Signup functions return the user's WebID. Sometimes users don't have a WebID account, and in that case they need to sign up for one. The signup process also results in getting the user's WebID. If the operation is successful and a WebID is returned, then the user is considered to be authenticated."

Not clear what the relationship is between signup and login.
The app can't tell whether the user has a webid up front and decide which to call.
The solid library should surely just allow login for those signed up and otherwise sign up, with the same result in case? So there is just one function to do both?

Also this function must be able (in future) to call the app back that the user has logged out (with a null webid).