> It actually does, but it has no side effect.

I know that printing is a side effect.  Why do you recommend to use
'alist-cons'?  If it's not possible to display anything, then
'roll-back' won't be able to print the following messages.

(simple-format #t "guix-package: switching from generation ~a to ~a~%"
               current-profile-number previous-profile-number)

(leave (_ (string-append
           "guix-package: previous profile doesn't exist; "
           "not rolling back~%")))

Actually, if the above is correct, then it won't be possible to use
'roll-back' at all.  (Because it creates symlinks.)

I think that I misunderstood you.  Please elaborate.

> When ‘--profile’ is passed, the result of ‘parse-options’ contains a
> pair whose key is ‘profile’.  The (assoc-ref opts 'profile) calls that
> you see retrieve the argument given to ‘--profile’.

It's clear.  But where should I call (assoc-ref opts 'profile)?  Should
I do it in 'process-query'?

Nikita