Fediz - Passing roles in wsclientWebapp web service

Hello.

I am working on the wsclientWebapp example of fediz. I have seen that that I
can get the name of the principal passed in the web service through
"WebServiceContext" and that is really cool. Though I have 2 questions:

1) How that mechanism work?
2) Is it possible to pass user's roles with the same mechanism. I tried for
example "context.isUserInRole("Admin")" but it didn't work.
I would like to have web services that work only if user has some specific
roles, so that operation would be really useful to me.

Thanks in advance,
Ioannis Samartzis.