object Server is users:Users and udb:UserDatabase
    description: (*
        A server has a list of authentic users and a database.
        Note that we are abstracting out server access because it is
        beyond the scope of this course.
    *);
end Server;

object Users is UserRecord*;
object UserRecord is uid:UserID and pw:Password;
object UserID is string;
object Password is string;
object UserDatabase is string;


operation Login
    inputs: s:Server, uid:UserID, pw:Password;
    outputs: udb:UserDatabase;

    description:(*
        Login to the given server to access its database.  If the user
        name and password exist on the server's user list, then return
        the database, otherwise nil.
    *);

    precondition:
        (*
         * User name and password combo must exist on server.
         *)
        exists (u in s.users) (u.uid = uid) and (u.pw = pw);

    postcondition:
        (*
         * If UserID and Password are valid, return database.
         *)
        udb = s.udb;

end Login;