Am Sonntag, 27. Januar 2013 schrieb Ludovic Courtès:
> ludo@gnu.org (Ludovic Courtès) skribis:
> > Having agreed on linear history, it seems that (a) the current
> > behavior is broken because roll-backs don’t actually follow the
> > history, as illustrated previously, and (b) the generation from which
> > we are rolling back must be deleted.

It seems to work: I rolled back from 21 to 20, 19, 18, 17; then removed a 
package and am at 18 now. Then removed another package and arrived at 19, 
where the previous 18 and 19 were overwritten.

Personally, I would have deleted all (consecutive) generations starting 
with 19 after the first roll-back and additional package removal; now we 
still have pieces of old history lying around, the (old and) current 20 is 
not a successor of the current 19 any more.

But I can also live with the current situation.

Am Sonntag, 27. Januar 2013 schrieb Ludovic Courtès:
> Andreas Enge <andreas@enge.fr> skribis:
> > $ guix-package --roll-back
> > error: no previous profile; not rolling back
> > 
> > No links are changed. I think in this case, rollback should create the
> > "empty profile" and have $PERUSER/guix-profile-1-link point to it.
> 
> Implemented in d930726.

It works also:
building path(s) `/nix/store/2gkfim0yry8sii7vhxwcivkbnfpaiqiq-user-
environment'
building user environment `/nix/store/2gkfim0yry8sii7vhxwcivkbnfpaiqiq-
user-environment' with 0 packages...
switching from generation 1 to 0

Thanks!

Andreas