I wrote:
> Here's a preliminary patch that does two things:
>
> * Changes 'guix-build' and 'guix-package --install' so that only the
>   newest packages will be considered (unless a version number is
>   specified).
>
> * Implements 'guix-package --upgrade'.
>
> Although I'm not aware of any functional problems with this code, I'm
> not entirely pleased with its organization.  Nonetheless, I wanted to
> make it available for early testing and comments.
>
> I welcome suggestions on how to improve this code.

Sorry, that patch had a problem in guix-build.  Here's a fixed version.

     Mark