Installing system with packages, sysconfig.scm syntax

[白い熊]

I'm trying to understand the sysconfig.scm syntax to specify when clean 
installing the system or running “guix system reconfigure”. 

In the manual I've seen the example with Emacs. But I don't understand why 
there's: 

(use-modules (gnu)
             (gnu packages emacs)

and then in the operating-system section

(packages (cons emacs %base-packages))

Let's just say I want to reconfigure the system with packages emacs, mc, 
xmodmap, and xterm installed for user joe. What do I put in the sysconfig.scm? 
I've tried many combinations, but can't accomplish this simple task. 

I understand I can install the system with a working sysconfig.scm and then 
install the packages manually via “guix package -i” but I think they all can be 
defined in sysconfig.scm, just can't figure out how, even though I'm trying to 
get hints in the manual. Or am I wrong in this? 

Also, when installing from the USB, after booting the USB, can I “guix pull” 
and then install the system? Or do I have to first install, then boot it, then 
“guix pull”, meaning I couldn't maybe put all package definitions in the 
install sysconfig.scm, as the packages might not be available before “guix 
pull”? 


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白い熊 @相撲道