enable completion on group subscription

[Giacomo Boffi]

let's say that i'm in my "*Group*" buffer, connected via nntp to a
functional newsserver

when i try to subscribe "U" a new newsgroup, completion gives me
_only_ completions for the groups i've already subscribed

of course i can browse "B" my newsserver just like a foreign one and
subscribe whatever group to my content, but i feel like gnus _should_
let me choose from the active file using completion

i suspect that i have to understand something more about groups'
levels, and probably it is in the manual, but gnus' manual and me were
maybe conceived on different planets: i'm unable to find the correct
solution!

after all this bla bla, my question boils down to:

what has to be done to enable completion on existing, available
newsgroups?

thanks in advance,
                                                                     g
-- 
> In tutti noi c'è un lato interista
Lato perlopiù nascosto dalle mutande.
                           --- Basil Fawlty, a reti unificate (IFQ+ISC)