Warning! Do not use this if you are not on the 'vbraun' branch of the sage-on-gentoo overlay.
By default you should be on the master branch.
The 'vbraun' branch is 
 * work in progress not yet deemed ready for general use
 * providing git snapshot for sage*-9999 ebuilds from sage's release manager (Volker Braun) merge branch.

This is the git repo where PR are merged in between develop release. 
Patches may be different from the master branch and at times broken because of PR(s) that may end up unmerged.

To minimize the difference between the master and vbraun branches, the git repo is now set
by overriding the ebuilds using the env/package.env mechanism.

Once on the vbraun branch put the files in place
ln -s /var/db/repos/sage-on-gentoo/tools/vbraun.settings/env.sagemath-vbraun \
      /etc/portage/env/sagemath-vbraun

ln -s /var/db/repos/sage-on-gentoo/tools/vbraun.settings/package.env.sagemath-vbraun \
      /etc/portage/package.env/sagemath-vbraun

You cannot use the vbraun branch of the overlay if you installed the overlay with 
`eselect repository enable sage-on-gentoo`, this method of installation use a Gentoo maintained mirror
of the overlay that does not include the vbraun branch. 
If you want to follow the vbraun branch, you need to follow the manual install method for the overlay.

To switch to the 'vbraun' branch of the overlay do
cd /var/db/repos/sage-on-gentoo/
git checkout vbraun
git pull

And to return to the master branch
cd /var/db/repos/sage-on-gentoo/
git checkout master
git pull

Do not forget to unlink the 'sagemath-vbraun` env and package.env files when returning to the master branch!

This mechanism only affects the -9999 ebuilds and not the sage releases.