> ‘%store’ is a SRFI-39 parameter (info "(guile) Parameters"), aka. a
> dynamically-scoped variable.

> It is initialized with the ‘parameterize’ form, which sets its value for
> the dynamic extent of its body.

I don’t understand what code initializes ‘%store’.

For example, I have one generation in the ‘test’ profile.  So ‘test’
points to ‘test-1-link’.  If I do

scheme@(guix scripts package)> (roll-back "test")

I get an error

guix/derivations.scm:428:17: In procedure derivation->output-path:
guix/derivations.scm:428:17: In procedure struct_vtable: Wrong type argument in position 1 (expecting struct): #f

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guix scripts package) [1]> 

This error is produced by (profile-derivation (%store) '()) since
‘%store’ is set to #f.

But why does this command work?

$ ./pre-inst-env guix package -p test --roll-back
switching from generation 1 to 0

As far as I understand, this code is called when the above command is
invoked:

    ;; First roll back if asked to.
    (if (and (assoc-ref opts 'roll-back?) (not dry-run?))
        (begin
          (roll-back profile)

I don’t see any references to ‘%store’ here.