inferiors and services

[Todor Kondić]

Dear guixians,

During the operating system instantiation -- either through `reconfigure`, or 
various `image` procedures) -- is the only possible way of controlling which 
version of services and the packages installable by those services to specify a 
particular commit in the channel config?

Inferiors help isolate a package manifest together with an user-controlled guix 
commit, so I was wondering if anything such is possible with the system itself 
(for example, during building of an image). In pseudocode:

(define inferior (inferior-from-channels ...))
(with-inferior inferior (reconfigure (operating-system ...)))

I am almost certain there is some gexp 
mumbo-jumbo-derivation-of-the-whole-system one can use, but not yet entirely 
comfortable with them.

T