I’d like to know how ‘roll-back’ creates an empty generation because
it’s necessary to do the same for ‘--delete-generations’.

However, I fail to understand how (profile-derivation (%store) ‘())
works (or any other function that uses (%store)).  I assume that some
code should set ‘%store’ to something else.  Like so:

scheme@(guix scripts package)> %store
$1 = #<<parameter> 934e7f8 proc: #<procedure 934e810 at ice-9/boot-9.scm:3154:17 () | (x)>>
scheme@(guix scripts package)> (%store)
$2 = #f
scheme@(guix scripts package)> (%store "FOO")
$3 = #f
scheme@(guix scripts package)> (%store)
$4 = "FOO"

But I don’t see anything similar in ‘package.scm’.