Hi,

I've slightly changed the 'roll-back' function. Now it prints an
informative message. And it's also possible to call it from a
command-line.

  # ./pre-inst-env guix-package --roll-back
  guix-package: switching from generation 6 to 5

The only problem I see is the following line.

  (error "there are no other profiles.")

It will print an ugly backtrace if there are no other profiles. I'm
trying to understand how to handle it in a non-ugly way.

Any comments are appreciated.

Nikita