A question about 'guix build -f'

[Feng Shu]

I want to build emacs-exwm-x in my guix-git repo with the following command:

   guix build emacs-exwm-x -f /home/feng/project/guix/gnu/packages/emacs.scm

But it seem to do nothing, any suggestion?


by the way, pre-inst-env is not suit my need, I just want to hack a
small package -> rebase it to latest git commit -> test build and install -> 
format patch

The problem is that, when I git pull to get latest git commit, test
build and install need download and build many many package, this is
veeeeeeery sloooooooooow. so I need a way I can code in guix git repo
and use system guix to test build and install.
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