To enable this repository, run:
apt-get install curl
curl -O https://www.afulinux.de/debian/dists/stretch-afulinux/afulinux-archive-keyring_2023.09.03.01652_all.deb
dpkg -i afulinux-archive-keyring_2023.09.03.01652_all.deb
To install a package from this repository, for instance e4rat-lite, run:
apt-get update
apt-get -t stretch-afulinux install e4rat-lite
Have fun!